Preserve upper and lower case for header keys.

[Isolus]

Even if it doesn't make any difference according to the standards, there is one in practice. For example, rspamd increases the spam score for Mime-Version (instead of MIME-Version). Some time ago the code in this repo was changed to set MIME-Version instead of Mime-Version. But the usage of net/textproto applies CanonicalMIMEHeaderKey on all header keys which changes MIME-Version back to Mime-Version. https://github.com/golang/go/issues/29965 suggets forking net/textproto.