parsing add case-sensitive headers with '@'

[GodKelvin]

Because in some applications the header is sensitive, for this, if I pass an '@', I make the same value in header, without capitalize. For example, I have a request for API make in C#, him break if I pass 'Token' but worked if I pass 'token', because he not follow the RFC.

[kahshing96]

hi @GodKelvin @nobu is there any update on this PR? would like to have this feature be merged which will be helpful in certain case where the 3rd-party API server is not following the RFC standard