Fix bug with '+' char on POST request encoding

Audioscrobbler was triggering error on the lastfm POST API in case of track with artist, album or title with a '+' in the string.

The '+' needs to be % escaped in the POST body otherwise Lastfm return a signature error. 

There may be problem with other characters too as only '=' '&' and '%' and now '+' are the only escaped character instead of properly url escaping the string but I was unable to found any other.