Fixed bearer token retrieval in conformity with RFC

I'm pretty sure the base64 decoding in the ResourceServer::determineAccessToken() method is a mistake.

According to the [7.1](http://tools.ietf.org/html/rfc6749#section-7.1) section of the [RFC6749](http://tools.ietf.org/html/rfc6749):

> The "bearer" token type defined in [RFC6750] is utilized by simply including the access token string in the request:
     GET /resource/1 HTTP/1.1
     Host: example.com
     Authorization: Bearer mF_9.B5f-4.1JqM