Ability for user defined Http headers whitelististing

[obonyojimmy]

Issue
Currently with server-connection Meteor-onConnection , there is only a small subset of whitelisted http-headers pass-through, this is a blocker where some user-defined http-headers are blocked , hindering logic which is dependant on this http-headers , a good example is when working with application tracing libs ie opentracing which is dependent on this headers for context propagation.

proposal
Customize Meteor-onConnection() , with probably enums of extra user-defined http-headers that are to passthrough.

[mitar]

Sadly, I think this a limitation of sock.js: sockjs/sockjs-node#202