Pull plain content type handling up to render

`render` is the only possible source for the `plain` option.  Pulling
the conditional up to the `render` method removes far away conditionals

actionpack/lib/abstract_controller/rendering.rb

@@ -23,7 +23,8 @@ module AbstractController
     def render(*args, &block)
       options = _normalize_render(*args, &block)
       self.response_body = render_to_body(options)
-      _process_format(rendered_format, options[:plain]) if rendered_format
+      _process_format(rendered_format) if rendered_format
+      self.content_type = Mime::TEXT if options[:plain]
       self.response_body
     end
 
@@ -99,7 +100,7 @@ module AbstractController
 
     # Process the rendered format.
     # :api: private
-    def _process_format(format, plain = false)
+    def _process_format(format)
     end
 
     # Normalize args and options.

actionpack/lib/action_controller/metal/rendering.rb

@@ -56,14 +56,10 @@ module ActionController
       nil
     end
 
-    def _process_format(format, plain = false)
+    def _process_format(format)
       super
 
-      if plain
+      self.content_type ||= format.to_s
-        self.content_type = Mime::TEXT
-      else
-        self.content_type ||= format.to_s
-      end
     end
 
     # Normalize arguments by catching blocks and setting them on :update.

actionview/lib/action_view/rendering.rb

@@ -104,7 +104,7 @@ module ActionView
       end
 
       # Assign the rendered format to look up context.
-      def _process_format(format, plain = false) #:nodoc:
+      def _process_format(format) #:nodoc:
         super
         lookup_context.formats = [format.to_sym]
         lookup_context.rendered_format = lookup_context.formats.first