gddo-server: use X-Appengine-User-Ip to identify ip of requests.

When running on GAE, the X-Real-Ip in header is shielded as well.
X-Appengine-User-Ip contains the real ip address in this situation.

This fixes golang/gddo#393.

Change-Id: I64fc4aa0a16a613a4ce4727aa1425d081ce1f0c9
Reviewed-on: https://go-review.googlesource.com/22067
Reviewed-by: Alan Donovan <adonovan@google.com>
1 file changed