IntoResponse implementation for Result<A: IntoResponse, B: IntoResponse>

[LukeMathWalker]

This closes #371 - it turns out there was already an implementation, it was just commented out :grin:

[dignifiedquire]

what if I want to return a redirect?

[dvic]

Redirects are one thing but I think it can also be perfectly fine to return HTTP 200's for errors. For example, often in GraphQL HTTP implementations one wants to return HTTP 200 for error responses as there is a distinction between success and failure responses in the serialization format.

[LukeMathWalker]

If there is consensus on the possibility of returning successes from the Err variant, I am happy to change the PR and simply cast both variants to Response without further checks.

[LukeMathWalker]

Updated the PR to avoid panic on the Err variant if it returns a success.