Ensure OAuth1 signature is created as per the spec

[AlexC]

[AlexC]

Update to replace another urldecode with rawurldecode ... fixes issues with signature that contains "+" (they were replaced with spaces)

[sblaz]

I think the 2 rawurlencode() calls on line 627 might be unnecessary. I was seeing a double-encoding of my oauth_callback parameter value and it appears an earlier call to normalize_parameters() takes care of things correctly.

[coderkevin]

In my case, normalize_parameters() is really messing up my already partially encoded parameters, so by removing normalize_parameters() instead, I was able to keep the rawurldecode() calls.

See: #91

I have created a PR which merges your changes with AlexC's and removes normalize_parameters() in favor of encoding them in the join_with_equals_sign() function: https://github.com/WP-API/OAuth1/pull/92