Change 500 reason phrase to "Internal Server Error"
Change 500 reason phrase to "Internal Server Error"

See https://github.com/onsip/SIP.js/issues/32#issuecomment-45348197

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