Co-authored-by: Matt Bostock <matt@mattbostock.com>
Co-authored-by: Jeremy Evans <code@jeremyevans.net>

The Request.forwarded_priority accessor sets the priority.  Default to
considering Forwarded first, since it is now the official standard.

Also allow configuring whether X-Forwarded-Proto or X-Forwarded-Scheme
has priority, using the Request.x_forwarded_proto_priority
accessor.

Allowing configurable priorities for these headers is necessary,
because which headers should be checked depends on the environment
the application runs in.

Make Request#forwarded_authority use the last forwarded authority
instead of the first forwarded authority, since earlier forwarded
authorities can be forged by the client.

Fixes rack#1809
Fixes rack#1829
Implements rack#1423
Implements rack#1832

Only used in one place and no interpolation, there is no reason to
use a constant in this case.