only call self.content_type= when there is a response

Apparently the AbstractController (whatever "abstract" means) is
expected to work without a request and response.

actionpack/lib/abstract_controller/rendering.rb

@@ -24,7 +24,11 @@ module AbstractController
       options = _normalize_render(*args, &block)
       self.response_body = render_to_body(options)
       _process_format(rendered_format) if rendered_format
-      self.content_type = Mime::TEXT if options[:plain]
+      if options[:plain]
+        _set_content_type Mime::TEXT.to_s
+      else
+        _set_content_type _get_content_type(rendered_format)
+      end
       self.response_body
     end
 
@@ -103,6 +107,13 @@ module AbstractController
     def _process_format(format)
     end
 
+    def _get_content_type(rendered_format) # :nodoc:
+      rendered_format.to_s
+    end
+
+    def _set_content_type(type) # :nodoc:
+    end
+
     # Normalize args and options.
     # :api: private
     def _normalize_render(*args, &block)

actionpack/lib/action_controller/metal/mime_responds.rb

@@ -191,6 +191,7 @@ module ActionController #:nodoc:
 
       if format = collector.negotiate_format(request)
         _process_format(format)
+        _set_content_type _get_content_type format
         response = collector.response
         response ? response.call : render({})
       else

actionpack/lib/action_controller/metal/rendering.rb

@@ -56,10 +56,12 @@ module ActionController
       nil
     end
 
-    def _process_format(format)
-      super
+    def _get_content_type(rendered_format)
+      self.content_type || super
+    end
 
-      self.content_type ||= format.to_s
+    def _set_content_type(format)
+      self.content_type = format
     end
 
     # Normalize arguments by catching blocks and setting them on :update.