Fix configuration load for parameters accepting (0, false, null) values.
Fix configuration load for parameters accepting (0, false, null) values.

Escaux/SIP.js,dpocock/JsSIP,Voiceworks/SIP.js,ronlobo/SIP.js,vconsult/JsSIP,DrumTechnologiesLtd/JsSIP,rusekr/JsSIP,markandrus/SIP.js,joseph-onsip/SIP.js,tqcenglish/JsSIP,josephfrazier/SIP.js,onsip/SIP.js,voxbone/JsSIP,onsip/SIP.js,AaronZhangL/JsSIP,CodeYellowBV/JsSIP