Improvements to 2xx retransmission behaviour
Improvements to 2xx retransmission behaviour

Fixes #131

joseph-onsip/SIP.js,Voiceworks/SIP.js,onsip/SIP.js,onsip/SIP.js,josephfrazier/SIP.js,Escaux/SIP.js,markandrus/SIP.js,ronlobo/SIP.js