HeaderName in .header is automatically converted to lower-case

[chirok11]

Hello!
While using Request builder .header is ignoring case in HeaderName and automatically converted to lower-case.
I created issue hyperium/http#535. and hoped to solve it through using .header method, but unfortunately - it is automatically ignoring my case and converting to lower-case header name.
I hope you make changes and at least willl stop forcing custom user headers to lower-case.

[seanmonstar]

This has been discussed in #402 and #924.

[rcastill]

Not exactly up for discussion