With xheaders request.remote_ip should return first ip in X-Forwarded-For chain

[mclate]

When request is proxied through several proxies, X-Forwarded-For header will have values in format origin,proxy1,proxy2,...
This PR will allow client to get first value in forwarded chain, thus getting ip of the origin request.

[bdarnell]

The first and last X-Forwarded-For values are both useful, but for different purposes. The last value is most analogous to the actual socket address used for remote_ip, so we should continue using it here. See #904 and #1051 for discussion about adding a method to return the first value.