net/http: make Transport respect non lower case Content-Encoding

net/http: make Transport respect non lower case Content-Encoding

The existing Transport implementation does not detect gzip encoding
when the Content-Encoding header is not lower-case. This is not
compliant with RFC2616 section 3.5 "All content-coding values are
case-insensitive." and caused issues in the wild.

Fixes #19248

Change-Id: I1b49992832dc3c8ef700058596a27dd9909640a3
Reviewed-on: https://go-review.googlesource.com/37431
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>