Allow http.Agent usage

[EugenDueck]

by passing in an http.Agent instance (either http.globalAgent or any custom instance) to the XMLHttpRequest constructor.

This addresses #44 and #48.

This is so that connections can be reused by the standard Http Keep-Alive mechanism, that is based on the http.Agent.maxSockets setting. Without agents, every request means a new TCP connections, behavior that differs from browser implementations of XHR.

One ramification of the former that made me implement this was that one TCP socket per HTTP request severely limits your ability to do load testing, as all your outbound ports get consumed in the TIME_WAIT state in no time. Try e.g. socket-io-client and the xhr-polling transport from node.js, which will use node-XMLHttpRequest, to verify.

Browser implementations of XHR on the other hand recycle their TCP sockets, otherwise socket-io based clients would bail as well in normal, non load-testing scenarios, when more than 16k events in 4 minutes are sent (the numbers highly depend on your OS and TCP settings).

Implementation note: It maintains backwards compatibility, in that if you don't pass in an agent, none will be used.

Ideally however, I'd like http.globalAgent to be the default, like it is with node's http requests, and like it was the case for node-XMLHttpRequest until changed by 8958631 , which @guile also laments in #44. However, my patch maintains the current status quo.

[TooTallNate]

+1

[TooTallNate]

For future-proof-ness, maybe we should make this be an "options" object with and agent property. That allows for more options to be added in the future without adding unnecessary arguments.

[jbeuckm]

Yes please. I'm about to send in an optional proxy as well.