A collection of general API improvements

- Working with Conn APIs requires calling new/0 or new/1 first
  old APIs are deprecated
- new/1 parses the provided string with URI.parse/1 and sets the Conn
  struct fields accordingly (so you can pass full or partial url
  fragments and get what you would expect, including those containing
  query strings)
- All headers are normalized to lower-case on put/get, and internally
  headers are a simple key/value map instead of the previous key =>
  key/value map, which was unnecessary.
- Implemented get_req_headers/1, deprecated get_req_header/1
- Implemented put_req_headers/2, deprecated put_req_header/2
- get_req_header/2 returns String.t instead of {String.t, String.t}
- get_req_headers/1 returns %{String.t => String.t} instead of
  %{String.t => %{String.t => String.t}}
- Added a number of typespecs, and fixed invalid ones
- Some small style fixes, documentation updates
- Fixed a bug with the Header middleware which was not overriding
  headers in the connection if already set. This one may be up for
  discussion if what's really desired.

lib/maxwell/adapter/hackney.ex

@@ -34,10 +34,9 @@ if Code.ensure_loaded?(:hackney) do
     end
 
     defp format_response({:ok, status, headers, body}, conn) when is_binary(body) do
-      headers = for {key, value} <- headers, into: %{} do
-        down_key = key |> to_string |> String.downcase
-        {down_key, {key, to_string(value)}}
-      end
+      headers = Enum.reduce(headers, %{}, fn {k, v}, acc -> 
+        Map.put(acc, String.downcase(to_string(k)), to_string(v)) 
+      end)
       %{conn | status:        status,
                resp_headers:  headers,
                req_body:      nil,
@@ -57,8 +56,6 @@ if Code.ensure_loaded?(:hackney) do
     defp format_response({:error, reason}, conn) do
       {:error, reason, %{conn | state: :error}}
     end
-
   end
-
 end
 

lib/maxwell/adapter/httpc.ex

@@ -66,10 +66,10 @@ defmodule Maxwell.Adapter.Httpc do
 
   defp header_serialize(headers) do
     {content_type, headers} = Map.pop(headers, "content-type")
-    headers = Enum.map(headers, fn({_, {key, value}}) -> {to_char_list(key), to_char_list(value)} end)
+    headers = Enum.map(headers, fn {key, value} -> {to_char_list(key), to_char_list(value)} end)
     case content_type do
-      nil -> {nil, headers}
-      {_, type} -> {to_char_list(type), headers}
+      nil  -> {nil, headers}
+      type -> {to_char_list(type), headers}
     end
   end
 
@@ -80,9 +80,7 @@ defmodule Maxwell.Adapter.Httpc do
   defp format_response({:ok, {status_line, headers, body}}, conn) do
     {_http_version, status, _reason_phrase} = status_line
     headers = for {key, value} <- headers, into: %{} do
-      key = key |> to_string
-      down_key = key |> String.downcase
-      {down_key, {key, to_string(value)}}
+      {String.downcase(to_string(key)), to_string(value)}
     end
     %{conn | status:        status,
              resp_headers:  headers,
@@ -107,6 +105,5 @@ defmodule Maxwell.Adapter.Httpc do
   defp format_response({:error, reason}, conn) do
     {:error, reason, %{conn | state: :error}}
   end
-
 end
 

lib/maxwell/adapter/ibrowse.ex

@@ -58,10 +58,9 @@ if Code.ensure_loaded?(:ibrowse) do
 
     defp format_response({:ok, status, headers, body}, conn) do
       {status, _} = status |> to_string |> Integer.parse
-      headers = for {key, value} <- headers, into: %{} do
-        down_key = key |> to_string |> String.downcase
-        {down_key, {key, to_string(value)}}
-      end
+      headers = Enum.reduce(headers, %{}, fn {k, v}, acc ->
+        Map.put(acc, String.downcase(to_string(k)), to_string(v))
+      end)
       %{conn | status:        status,
                resp_headers:  headers,
                resp_body:     body,
@@ -77,8 +76,6 @@ if Code.ensure_loaded?(:ibrowse) do
     defp format_response({:error, reason}, conn) do
       {:error, reason, %{conn | state: :error}}
     end
-
   end
-
 end
 

lib/maxwell/adapter/util.ex

@@ -31,19 +31,17 @@ defmodule Maxwell.Adapter.Util do
   end
 
   @doc """
-  Make headers to key word list.
+  Converts the headers map to a list of tuples.
 
-     * `headers`   - `Map.t`, for example: `%{"content-type" => {"Content-Type", "application/json"}}`
+     * `headers`   - `Map.t`, for example: `%{"content-type" => "application/json"}`
 
   ### Examples
 
-       #[{"Content-Type", "application/json"}]
-       iex> headers_serialize(%{"content-type" => {"Content-Type", "application/json"}})
-
+       iex> headers_serialize(%{"content-type" => "application/json"})
+       [{"content-type", "application/json"}]
   """
-
   def header_serialize(headers) do
-    headers |> Map.values
+    Enum.into(headers, [])
   end
 
   @doc """
@@ -83,9 +81,8 @@ defmodule Maxwell.Adapter.Util do
   """
   def chunked?(conn) do
     case Conn.get_req_header(conn, "transfer-encoding") do
-      {_, "chunked"} -> true
-      {_, type} -> "chunked" == String.downcase(type)
-      nil -> false
+      nil       -> false
+      type      -> "chunked" == String.downcase(type)
     end
   end
 
@@ -122,15 +119,15 @@ defmodule Maxwell.Adapter.Util do
     case next_stream_fun.({:cont, nil}) do
       {:suspended, item, next_stream_fun} -> {:ok, item, next_stream_fun}
       {:halted, _} -> :eof
-      {:done, _} -> :eof
+      {:done, _}   -> :eof
     end
   end
   def stream_iterate(stream) do
-		case Enumerable.reduce(stream, {:cont, nil}, fn(item, nil)-> {:suspend, item} end) do
+    case Enumerable.reduce(stream, {:cont, nil}, fn(item, nil)-> {:suspend, item} end) do
       {:suspended, item, next_stream} -> {:ok, item, next_stream}
-			{:done, _} -> :eof
+      {:done, _}   -> :eof
       {:halted, _} -> :eof
-		end
+	end
   end
 
   defp multipart_body({:start, boundary, multiparts}) do