Remove content-length header because it messes with some proxy configs

[AshKyd]

Submitting for your consideration, though I totally understand if it's rejected:

• I've been having trouble getting Loggly to log anything through a series of corporate proxies in our environment. The request module automatically picks up the https_proxy environment variable, and so all should be good.
• I haven't worked out exactly why, but when the content-length is set, one of the two proxies I'm going through hangs until the request eventually times out. In the interim, the callback doesn't fire and I suspect it sits there leaving the connection open because eventually (with enough .log() calls) it'll crash one of the proxies.
• Removing the content-length makes everything work :+1: though I can only guess as to why.

I can sort of guess why the content-length header it's there, but I'm just wondering how necessary it is and whether we can safely ditch it for compatibility reasons?

[indexzero]

Not opposed to this being an option, but it needs to be implemented in a more generic way, not just removed wholesale.