Add test that X-Forwarded-Proto header is used when behind a proxy

Port of test from Dancer PR #1015.
Update PSGI env keys with 'HTTP_' prefix for context and error tests
Client supplied HTTP headers are prefixed with 'HTTP_' as per PSGI spec.

PSGI spec applies a transform to client-supplied headers like X-Forwarded-For.
Ensure that we only look for the 'HTTP_' prefixed headers in the $env hashref.
PSGI spec transforms Cookie header to HTTP_COOKIE key in PSGI env.

Cookie handling is now (well, again) similar to what Plack::Request does.
Update Request pod to better explain PSGI env and headers
reflect change
Merge branch 'pr/psgi_header_cleanup'