Session: on re-INVITE, send 488 instead of calling this.receiveReinvite

See https://github.com/onsip/SIP.js/issues/19#issuecomment-43887857
Merge pull request #50 from joseph-onsip/rejectReinvite

Session: on re-INVITE, send 488 instead of calling this.receiveReinvite

fixes #19