Improve error when redirecting with custom schemes

[jcugat]

Fixes #822

I used Scheme "blah" not supported as the error message instead of URL scheme must be "http" or "https". used in other parts of the code. It makes more sense to show the actual invalid scheme since it's coming from the server and not something that the user specified.

I also tried the same test with allow_redirects=False but unfortunately when the response object is built, it needs a port which we can't derive from the custom scheme. Not sure how to solve this without changing the port definition of Url to allow a None port, ideas?

[jcugat]

This pattern ("http", "https") (or something similar) is repeated in a few places, like:

https://github.com/jcugat/httpx/blob/9160631d7fb957a319ce8985c97a858f8eab49a3/httpx/_client.py#L609
https://github.com/jcugat/httpx/blob/9160631d7fb957a319ce8985c97a858f8eab49a3/httpx/_models.py#L105
https://github.com/jcugat/httpx/blob/9160631d7fb957a319ce8985c97a858f8eab49a3/httpx/_models.py#L132

It would probably make sense to unify it in a single place.

[lovelydinosaur]

In these kinds of cases, if it's a single-liner I'd often prefer not to introduce indirection. It's as clear as it can possibly be, and it's not a complicated expression.

[florimondmanca]

Thanks! Sounds like an about right fix to me, waiting for an overhaul of scheme handling #954 and possibly mounting transports #977.

Happy to have someone else's opinion on this though :-)

[yeraydiazdiaz]

LGTM, thanks @jcugat!