Allowing various anti-spoofing headers for outgoing requests

[lucacasonato]

In the browser, accept-charset, date, dnt, origin, referer, via, proxy-*, sec-*, x-http-method, x-http-method-override, and x-method-override are disallowed because they should not be spoofable by users.

On the server however, these are not security sensitive, so they do not have to be stripped from outgoing requests.

[andreubotella]

Access-Control-Request-Headers and Access-Control-Request-Method are also CORS-related headers which shouldn't be stripped on the server.