c2s: only offer starttls if not encrypted already
c2s: only offer starttls if not encrypted already

eelcocramer/node-xmpp,stliu/node-xmpp,agebrock/node-xmpp,xmppjs/xmpp.js,qbit/node-xmpp,xmppjs/xmpp.js,WeBest/node-xmpp,tempbottle/node-xmpp,agebrock/node-xmpp,node-xmpp/node-xmpp,node-xmpp/node-xmpp,ggozad/node-xmpp,ggozad/node-xmpp,qbit/node-xmpp