[FEAT] Set Access-Control-Expose-Headers

[dotnetCarpenter]

In my current project I need to set Access-Control-Expose-Headers: Content-Length

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Expose-Headers

@jfhbrook I'm currently swamped at work, so would like to postpone our hackathon to beginning of February. I plan to take a swing at #209, #193, #154, #88 and mime types.

[jfhbrook]

no worries timing-wise, my personal life has been a little nuts lately and I probably won't have bandwidth until mid-feb anyway