Allow `send_file` to declare a charset

Removed my patch in favor of @tenderlove's less invasive approach.

[Aaron Patterson & Jon Moss]

actionpack/lib/action_controller/metal/data_streaming.rb

@@ -70,6 +70,7 @@ def send_file(path, options = {}) #:doc:
         send_file_headers! options
 
         self.status = options[:status] || 200
+        self.content_type = options[:type] if options.key?(:type)
         self.content_type = options[:content_type] if options.key?(:content_type)
         response.send_file path
       end

actionpack/lib/action_dispatch/http/response.rb

@@ -224,8 +224,10 @@ def status=(status)
 
     # Sets the HTTP content type.
     def content_type=(content_type)
-      header_info = parse_content_type
-      set_content_type content_type.to_s, header_info.charset || self.class.default_charset
+      return unless content_type
+      new_header_info = parse_content_type(content_type.to_s)
+      prev_header_info = parse_content_type(get_header(CONTENT_TYPE))
+      set_content_type new_header_info.mime_type, new_header_info.charset || prev_header_info.charset || self.class.default_charset
     end
 
     # Sets the HTTP response's content MIME type. For example, in the controller
@@ -403,8 +405,7 @@ def cookies
     ContentTypeHeader = Struct.new :mime_type, :charset
     NullContentTypeHeader = ContentTypeHeader.new nil, nil
 
-    def parse_content_type
-      content_type = get_header CONTENT_TYPE
+    def parse_content_type(content_type = get_header(CONTENT_TYPE))
       if content_type
         type, charset = content_type.split(/;\s*charset=/)
         type = nil if type.empty?