Add `simple_bridge:host/1` callback

[dmsnell]

Status

I know nothing about what I'm doing here and I didn't test any of this. What I've put together represents a draft idea for supporting a new interface to get the host data out of simple_bridge. I've only used cowboy and wouldn't say I'm an expert on how it operates or handles the host value (though I hope to experiment and test to find out).

Would like some review of the general idea and approach. See #79 for a motivating example: nitrogen_core crashes when calling wf:url() on a cowboy TLS server because it doesn't provide a host HTTP header. A cursory glance around the web turns up that Golang's standard library lifts the host header out of the headers and into a request property there as well. some part of Python's requests might omit the Host HTTP header in some cases but I didn't look deep enough to confirm.

Description

The host value for a given request may depend on multiple values:

• the Host HTTP header
• the X-Forwarded-For HTTP header
• the absolute URI for the request
• the Host property of the request set by the HTTP server

When applications rely on getting this value diretly through the HTTP headers they may run into issues with this being undefined or wrong. Possible scenarios may include:

• pre-HTTP/1.1 requests without an explicit Host HTTP header
• TLS requests when the underlying web server doesn't set a header from the initial :authority parameter.
• proxied requests with an X-Forwarded-For HTTP header

In this patch we're adding a new host/1 callback so that the underlying implementation can provide a proper value for the request's host, whether or not it's set as an HTTP header, and whether or not a proxy might be providing more accurate information.

[choptastic]

This is great stuff. This and your other PR were finally merged. I made a few small changes, but nothing major, and I added some tests.

One thing I didn't add a test for was the TLS or the X-Forwarded-For header, but I really like your suggestion for it.