fix(Response): Headers are case-sensitive

Web developers tend to think of headers being case-insensitive. However,
if you try to set the same header twice, with differently-cased header
names, Falcon treated them as separate headers.

This patch changes falcon.Request such that it now normalizes header
names internally, and when performing lookups.

Applications shouldn't break due to this change unless they were
accessing the private req._headers attribute directly.

Closes #185
perf(Request): Avoid repeated calls to self