added a check for x-host header on CDN use

[dnmitev]

We have found out that the ASP.NET MVC middleware is failling with wrong url when there is CDN, The X-Host header provided by the CDN was not taken into consideration so I have applied a check for that

[thoop]

Which CDN's use X-Host? I haven't seen that header added before.

[dnmitev]

@thoop we are using the Azure CDN and we have the problem there that the prerender.io is providing the url of the traffic manager instead of using the X-Host header which is the correct one. For an instance, if we have the address www.my-address.com which uses the my-traffic-manager.manager.com, the X-Host would containt "my-address.com" while the prerender.io would provide my-traffic-manager.manager.com. In other words we would have https://my-traffic-manager.com/users indexed instead of http://my-address.com/users

[thoop]

Ah, I see. Instead of using the X-Host header, would it be better to include an option for you to set in the middleware config to override the incoming Host to whatever you set in the middleware Host configuration?

That way everyone would be able to use it instead of just the people on Azure that have the X-Host header coming in. It's also a little more secure to use a config override instead of trusting the incoming Host when proxying.

[dnmitev]

@thoop I have applied the x-host header use from the config. Otherwise the x-host header is standard for CDNs as far as I know, so it's not just for Azure.