Add Accept-Post to Access-Control-Expose-Headers

[csarven]

Required in order to expose a non-simple header for CORS.

[dmitrizagidulin]

I thought about adding that header to Access-Control-Expose-Headers, in my original PR. And then decided against it -- the Access-Control-... header only applies to non-OPTIONS requests. Whereas the Accept-Post header is only returned with OPTIONS. Hence, we don't need it.

[csarven]

Consider:

Accept-Post is a MUST in OPTIONS in LDP, but it is not strictly for OPTIONS. It can be present in other methods.

Browsers still throw a Refused to get unsafe header if Accept-Post is not present. There is no harm in adding this as far as I can tell.

[dmitrizagidulin]

it's only implemented to return with OPTIONS, tho...