Fix X-Forwarded-Host when multiple proxies, see PR#928

[JonathanHuot]

See initial bugreport in the PR #928

This should fix the case where bottle is behind multiple reverse-proxy, and each proxy is adding its own X-Forwarded-Host.
When the request arrives at the bottle level, we have multiple values in X-Forwarded-Host: www.foo.com, xyz.foo.local. It makes any uses of request.url/bottle functions not possible.

E.g.
With X-Forwarded-Host: www.foo.com, xyz.foo.local given:

Before the patch:
request.url == 'http://www.foo.com, xyz.foo.local/index.html'

After the patch:
request.url == 'http://www.foo.com/index.html'

[JonathanHuot]

Any news on this ? Travis says the build is "pending" but it has been successfully completed.

[defnull]

I'm still convinced that a multi-valued X-Forwarded-Host header is a configuration error in the reverse-proxy or load-balancer setup, and does not make any sense. The header should contain the original Host as sent by the client, so only the first public-facing proxy should set this header and any additional back-end hops should completely ignore it.

Note that X-Forwarded-Host is different from X-Forwarded-For, wihc is multi-valued and already recognized as such by bottle.