Grammar - join escaped characters into string

When parsing SIP messages that contain NameAddrHeaders with parameters involving escaped character sequences (for example, `"Hiro" <sip:hiro@example.com;altsipaddr=hiro%40someother.com>`), JsSIP is misinterpreting the escaped character sequence as an array.  The parameter value is then joined (`pvalue.join('')`), which calls `toString` on the array.  The resulting NameAddrHeader has a uri looking like this: `"Hiro" <sip:hiro@example.com;altsipaddr=hiro%,4,0someother.com>`.

This pull requests fixes the issue by joining the escaped characters into a single string (rather than leaving as an array) after they have been parsed.