fixed a bug that would incorrectly decode strings with %2B (+)

The upstream library that implements `urlDecodedString` currently has a bug. It decodes the url first and then replaces "+" (plus) with a " "(space).  This interferes with strings containing %2B (+ in hex form).  Replacing it with `stringByReplacingPercentEscapesUsingEncoding:` until underlying library fixes the implementation.