do not create extenstion symbol if extension does not exist

Author: flyerhzm

lib/grape/middleware/formatter.rb

@@ -42,10 +42,10 @@ def before
 
       def format_from_extension
         parts = request.path.split('.')
-        extension = parts.last.to_sym
 
-        if parts.size > 1 && content_types.key?(extension)
-          return extension
+        if parts.size > 1
+          extension = parts.last.to_sym
+          return extension if content_types.key?(extension)
         end
         nil
       end