According to the Apache mod_proxy docs, X-Forwarded-Host can be a
comma-separated list of hosts, to which each proxy appends the requested
host. We want to grab only the first from this comma-separated list
to get the original requested Host when building URLs.
Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
628c7fa77b