Refactor and support asking for String?

spec/pg/driver_spec.cr

@@ -105,6 +105,10 @@ describe PG::Driver do
       db.query "select email from users limit 1" do |rs|
         assert_single_read rs, String, "foo@example.com"
       end
+
+      db.query "select email from users limit 1" do |rs|
+        assert_single_read rs, String?, "foo@example.com"
+      end
     end
   end
 

src/pg/result_set.cr

@@ -93,13 +93,24 @@ class PG::ResultSet < ::DB::ResultSet
   end
 
   def read(t : String.class) : String
-    value = read
-    if value.is_a?(String)
+    value = read(String | Slice(UInt8))
+
+    case value
+    when Slice(UInt8)
+      String.new(value)
+    else
       value
-    elsif value.is_a?(Slice(UInt8))
+    end
+  end
+
+  def read(t : String?.class) : String?
+    value = read(String | Slice(UInt8) | Nil)
+
+    case value
+    when Slice(UInt8)
       String.new(value)
     else
-      raise "#{self.class}#read returned a #{value.class}. A String was expected."
+      value
     end
   end