Use type alias for strict response types

[dan-j]

See #970...

@gerhardwagner suggested a fix which also works for me, so I've created a quick PR. I'm not familiar with the actual codebase so not sure if tests are going to pass, or how to add tests to get this merged.

I don't have a lot of time to go digging so any hints are appreciated and I am happy to contribute tests.

[gerhardwagner]

would like to see this merged 🙃

[dan-j]

Just added a comment to the issue #970 which might prevent this from being merged as-is as it could break other people's implementations