Multiple reverse proxy support

Apache for one allows the X-Forwarded-Host to be a comma seperated
list rather than a single value. See:
http://httpd.apache.org/docs/2.2/mod/mod_proxy.html#x-headers

In a case of:

[interwebs] -> [external proxy] -> [internal proxy] -> [Dancer]

The X-Forward-Host will look like "domain.example, domain.example". This will then be used in, e.g. redirects:

Location: http://domain.example, domain.example/redirected_page