Set Request Headers

[bjacquet]

I have a similar use case as #705, however in mine I need to set a Referer header on every request. 705's workaround didn't work for me and couldn't find any way of setting the header.

Maybe for simple use cases like mine having Wallaby.HTTPClient.headers/0 as a public function would be just enough to get me going. However I understand that "simply" changing to a public function can give rise to other issues.

Can you consider adding some way for specifying request headers in the future?

Thanks for this great software! 👍🏼