Document, refactor and create test case for ActionDispatch::Response#charset= method

actionpack/lib/action_dispatch/http/response.rb

@@ -187,13 +187,13 @@ module ActionDispatch # :nodoc:
       @content_type = content_type.to_s
     end
 
-    # Sets the HTTP character set.
+    # Sets the HTTP character set. In case of nil parameter
+    # it sets the charset to utf-8.
+    #
+    #   response.charset = 'utf-16' # => 'utf-16'
+    #   response.charset = nil # => 'utf-8'
     def charset=(charset)
-      if nil == charset
-        @charset = self.class.default_charset
-      else
-        @charset = charset
-      end
+      @charset = charset.nil? ? self.class.default_charset : charset
     end
 
     # The response code of the request.

actionpack/test/dispatch/response_test.rb

@@ -42,6 +42,13 @@ class ResponseTest < ActiveSupport::TestCase
     assert_equal Encoding::UTF_8, response.body.encoding
   end
 
+  def test_response_charset_writer
+    @response.charset = 'utf-16'
+    assert_equal 'utf-16', @response.charset
+    @response.charset = nil
+    assert_equal 'utf-8', @response.charset
+  end
+
   test "simple output" do
     @response.body = "Hello, World!"