Header Case Sensitive

[wbaridon]

Problem Description

Some server based on HTTP/1.1 don't respect the non sensitive condition for headers.
I get an error with the header authorization instead of Authorization.

In the past a similar fix have been deployed on an other proxy here:
mitmproxy/mitmproxy#1214

Steps to reproduce the behavior:

1. Call on a server HTTP/1.1 where we need to respect the uppercase