Add header configuration helper to simplify header setting

[spinscale]

This introduces a Headers class with a few pre-defined header names to
have a DSL to easily configure headers. This has been created with the
OWASP security headers in mind, but can be used for anything.

Closes #1246

[spinscale]

sorry for any kotlins embarassments :-) happy to reiterate on this

[tipsy]

Great work @spinscale, I left a few comments.

[spinscale]

one more thing to discuss might be to make this as an app.before handler, because that would allow handlers to overwrite these headers... I don't expect we need to protect the user, by not allowing to set those within handlers.

[tipsy]

I think app.before is better, and I also think .globalHeaders is a better function name than just headers.

[spinscale]

renamed to globalHeaders, changed to app.before and removed the null check

[tipsy]

Looks good from my end! Do you want to make more changes, or are you ready to merge?

[spinscale]

ready to merge!

[tipsy]

Perfect, thank you very much!