Header case sensitivity

[rufoa]

Hi @arrdem,

I've noticed that in your fork of this project, you've added code to look headers up in a case-insensitive manner.

According to the Ring spec, header names in the request map are downcased automatically. Have you come across a scenario in your use of my project where this does not work as expected?

If this is an actual bug (rather than a workaround for something weird you're doing internally at FC) please let me know so I can fix the problem upstream for the benefit of other people who use this library.

Thanks!