Make the response's request parameter optional

[tbascoul]

Make the response's request parameter optional

Part of #1227

[lovelydinosaur]

Okay, so it turns out this issue is actually more awkawrd than realised at first, because although we do want request=... top be optional on a Response() instance, we don't want it to be optional on RequestError/HTTPError exceptions, where we know it will always be set.

Here's what I think we're going to need to do here...

• The signatures on our exception classes should not change.
• The decoders should not raise DecodeError, or accept a request instance on __init__. Instead any decoding errors should just raise a plain old ValueError exception.
• The Response instance should wrap any decoding codeblocks instead a try ... except ValueError as exc. If response._request is set then the exception should be re-raised using DecodeError(message=str(exc), request=self.request) from exc, but if response._request is not set, then the plain ValueError exception should just be re-raised, using raise exc.

That'll allow us to have...

• An optional request=... parameter on the Response.__init__().
• Without changing the exception signatures.
• Properly raising httpx.DecodeError() on exceptions that occur during a request flow, where we will have an associated request instance.
• For unit-test style response instances, that do not have a request=... instance set, we'll get plain ValueError exceptions for any decoding errors.

You're welcome to take on these follow ups yourself, or else I'd also be happy to take a look at them.

[tbascoul]

Thanks for the detailed guidance.
Still some work to do on the testing part.

[tbascoul]

Raising a ValueError inside raise_for_status seems a bit off ?

[lovelydinosaur]

I'd suggest we start the method with something like...

if self._request is None:
    raise RuntimeError("Cannot call `raise_for_status` as the request instance has not been set on this response.")

[lovelydinosaur]

Great, yup! Thanks so much for your time on this!

I've added in a _wrap_decode_errors context manager, to remove some of the replication, and get the test coverage back up to 100%