Fix typespecs for some functions

[RobinBoers]

Hi, I was using this library and dialyzer started to complain:

lib/dummy/dummy.ex:94:call
The function call will not succeed.

Req.put([
  {:body, binary()} | {:headers, [{<<_::72>>, _}, ...]} | {:url, <<_::8, _::size(1)>>},
  ...
])

will never return since the 1st arguments differ
from the success typing arguments:

(
  binary()
  | %{
      :__struct__ => Req.Request | URI,
      :adapter => (... -> any),
      :authority => URI.authority(),
      :body =>
        nil
        | binary()
        | maybe_improper_list(
            binary() | maybe_improper_list(any(), binary() | []) | byte(),
            binary() | []
          ),
      :current_request_steps => _,
...

So I took a look at the source code, and saw that for some functions the keyword() type was missing in the typespec, while all those functions had a clause to handle keyword lists, and the docs clearly said a keyword list was accepted.

So I added them, and also made the docs for get/2 consistent with the rest.

[wojtekmach]

Thank you!