Fixed Response::RaiseOctokitError#error_message

The response may be decoded already. See issue #39 for a sample of this
bug.

lib/faraday/response/raise_octokit_error.rb

@@ -31,7 +31,7 @@ def on_complete(response)
 
     def error_message(response)
       message = if body = response[:body]
-        body = ::MultiJson.decode(body)
+        body = ::MultiJson.decode(body) if body.is_a? String
         ": #{body[:error] || body[:message] || ''}"
       else
         ''