Duplicate the default middleware stack

Duplicate the default middleware stack so that it won't be modified.

Author: akihikodaki

lib/octokit/default.rb

@@ -147,7 +147,7 @@ def login
       # from {MIDDLEWARE}
       # @return [Faraday::RackBuilder or Faraday::Builder]
       def middleware
-        MIDDLEWARE
+        MIDDLEWARE.dup
       end
 
       # Default GitHub password for Basic Auth from ENV