fixed a bug that incorrectly decoded %2B (+) characters

This bug manifests itself when the string we're decoding contains a %2B, which is a "+" (plus) character. References #7 and rsieiro/RSOAuthEngine#18 

Note, besides testing for fixing my failing case, I have not had the chance to test this and I don't understand the reasons behind chosing C-level functions instead of built-in method. If my approach is unacceptable I can update the fix by converting "+" into " " first then running it through the current `CFURLCreateStringByReplacingPercentEscapesUsingEncoding` method.