return InvalidArgument for non-103 1xx status codes

[joeshaw]

Add an integration test for this. With the Rust SDK this results in a
panic and a 500 response, which we check for in the test.

[bahamat]

My philosophy when I implemented this was that 1xx other than 103 would continue to exhibit whatever behavior it had before I made changes.

I'm not saying this change shouldn't be made, necessarily. But that was why I had decided not to add additional handling.

[joeshaw]

That makes sense! My motivation for opening this is to make the behavior consistent with Compute. Otherwise it makes testing harder, especially in the SDKs. For example, fastly/compute-sdk-go#218 and specifically https://github.com/fastly/compute-sdk-go/blob/54e5d57017e61504bc7500725f68e7c0428dfe8d/end_to_end_tests/loopback/main_test.go#L66-L87

[ulyssa]

Thank you for implementing this! Yes, mirroring Compute's behaviour here is the right thing to do. If we ever support other 1xx codes beyond 103, then we can update this to allow those through in the same way.