Ensure templates are rendered if all the parts are already processed

actionmailer/lib/action_mailer/base.rb

@@ -470,8 +470,9 @@ def create!(method_name, *parameters) #:nodoc:
         # also render a "normal" template (without the content type). If a
         # normal template exists (or if there were no implicit parts) we render
         # it.
-        template = template_root["#{mailer_name}/#{@template}"]
-        @body = render_message(@template, @body) if template
+        template_exists = @parts.empty?
+        template_exists ||= template_root["#{mailer_name}/#{@template}"]
+        @body = render_message(@template, @body) if template_exists
 
         # Finally, if there are other message parts and a textual body exists,
         # we shift it onto the front of the parts and set the body to nil (so