HTTP headers should be considered case-insensitive.

I found this when writing some tests for the decoder in libprocess.

Message header names should be case-insensitive:
http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2

Creating this issue to track it, I'm going to add some TODOs for now.
Most clients tend to use Camel-Case for the headers so this is not urgent.