push content_type assigment in to metal

everything above metal really doesn't care about setting the content
type, so lets rearrange these methods to be in metal.

actionpack/lib/abstract_controller/rendering.rb

@@ -24,9 +24,9 @@ def render(*args, &block)
       options = _normalize_render(*args, &block)
       self.response_body = render_to_body(options)
       if options[:html]
-        _set_content_type Mime::HTML.to_s
+        _set_html_content_type
       else
-        _set_content_type _get_content_type(rendered_format)
+        _set_rendered_content_type rendered_format
       end
       self.response_body
     end
@@ -106,10 +106,10 @@ def _process_options(options)
     def _process_format(format)
     end
 
-    def _get_content_type(rendered_format) # :nodoc:
+    def _set_html_content_type # :nodoc:
     end
 
-    def _set_content_type(type) # :nodoc:
+    def _set_rendered_content_type(format) # :nodoc:
     end
 
     # Normalize args and options.

actionpack/lib/action_controller/metal/mime_responds.rb

@@ -191,7 +191,7 @@ def respond_to(*mimes)
 
       if format = collector.negotiate_format(request)
         _process_format(format)
-        _set_content_type _get_content_type format
+        _set_rendered_content_type format
         response = collector.response
         response ? response.call : render({})
       else

actionpack/lib/action_controller/metal/rendering.rb

@@ -56,12 +56,14 @@ def _render_in_priorities(options)
       nil
     end
 
-    def _get_content_type(rendered_format)
-      self.content_type || rendered_format.to_s
+    def _set_html_content_type
+      self.content_type = Mime::HTML.to_s
     end
 
-    def _set_content_type(format)
-      self.content_type = format
+    def _set_rendered_content_type(format)
+      unless response.content_type
+        self.content_type = format.to_s
+      end
     end
 
     # Normalize arguments by catching blocks and setting them on :update.