caddyhttp: Canonicalize header field names

[ghostwheel42]

This fixes #5175 by canonicalizing the header field names.

[CLAassistant]

All committers have signed the CLA.

[mholt]

Thanks for the patch!

I just realized this is very similar to 2153a81.

[francislavoie]

Seems like this is just a fix for JSON, because the headers don't get set through .Add() and such as they do when unmarshaling the Caddyfile.

Should this be done at provision time instead of at runtime, to save some cycles?

[mholt]

@francislavoie That's a good idea -- I thought about it too, but it has to be done on the output of the replacer, which depends on the request. So I think we have to do it here.

[ghostwheel42]

@francislavoie I've thought about using the appropriate methods for this and/or doing it when provisioning, but as @mholt said it needs to be done on the output of the replacer and to use .Add() you'd need to iterate over the list of values which would probably be slower in the end.