Bugfix: withHeaders should merge headers rather than replace

[haddowg]

It is currently impossible to set custom headers on a Resource class response without a custom controller since the specific response object's headers are replaced by the wrapping DataResponse object.

This PR changes the behaviour of witHeaders to merge rather than replace.

BREAKING CHANGE: direct use of withHeaders will now merge rather than replace this may lead to unexpected results

[haddowg]

the more I think about it, i am not sure this would/should be considered breaking. Since the only headers added by the package are the content type and location when required on a 201, if you were calling withHeaders without setting these yourself then having them added should definitely not be breaking.

[lindyhopchris]

Thanks for the PR! I think this is probably safe to merge as non-breaking. I'm doing JSON:API things Weds evening so will check it over and merge/tag then.

[lindyhopchris]

I'm going to merge this as a non-breaking change, as I believe it's a bug - it should be merging the headers rather than replacing them.

I'll add a withoutHeaders() method so that people can remove any headers they want if they've been relying on this behaviour.

Thanks again for the PR.

[lindyhopchris]

I've tagged this as 4.2.0