Pr/psgi header cleanup

The PSGI spec states that client-supplied request headers get uppercased, hyphens replaced with underscores and prefixed with `HTTP_`.  After reviewing #566, there were several places the Request class (and tests) were using non `HTTP_` prefixed keys in the PSGI env. 

Specifically, all the `X-*` headers, and the `Cookie` header have the HTTP_ prefix added. Dancer2::Core::Request only need to check these prefixed keys in the env. (This Pr includes @racke tests from #566)

Also cleaned up the pod a little; explaining that the PSGI spec transforms headers, and using lowercase `\%env` in a couple of places, since uppercase %ENV has a different meaning..