net/textproto: allow manually register canonicalMIMEHeaderKey

[j16sdiz]

This is a feature request related to #18476 #18196 #5022 #18495.
Instead of controlling each request/response, may we have a function to register my own canonicalMIMEHeaderKey ?

The HTTP problem "will be" solved with HTTP/2, but MIME is more then just HTTP.
Instead of one-header at a time, may be we can have something easier to override?

[bradfitz]

What's the problem?

[j16sdiz]

Legacy, standard noncompliance client that do header in case sensitive way?

I am doing a mitm proxy for a binaryonly app to inject a auth header for our internal firewall.
When I get the header, the case is not preserved and broke the legacy app.

Apache mod_proxy, nginx, squid all do that in case preserving way. But there is no way to do this in go without rewriting the whole Round Tripper.

[j16sdiz]

I saw the previous proposal to control the case of header have been rejected.
That's why I am proposing an alternative way to do this -- allow adding my own entry to the hardcoded list.

[ianlancetaylor]

We aren't going to add new API to the standard library to support legacy clients that are not standards compliant. You will have to make your own copy of the net/textproto package, or run a transformation on the input stream.