Add Header-Case

[abemedia]

What does this fix or improve?

Adds the default casing used in HTTP headers i.e. capitalised snake case. I realise this can easily be achieved with ToCase, however this is a common casing so I felt it deserved first-class support.

Also fixed Makefile to use the newer go install command.

Is this a breaking change API change or change in behavior?

No

Checklist:

• I have updated the documentation accordingly, if needed.
• I have added appropriate tests, if needed.

[liyanchang]

Hi! Glad you're finding this library helpful!

I'm inclined to just leave it as is for the moment - as you mentioned, it's easy enough to just use ToCase. Obviously, I'm open to adding it if there's more interest from a wide range of users.

I appreciate the thoughtfulness you had in making sure you added tests, docs, etc. I'm definitely open to PR that's limited to just docs and makefile changes.

Thanks!