Support multiple content types per path

When attempting to specify two routes with the same path and method, but different accepted format (e.g. `format = "application/json"` vs `format = "application/cbor"`) error message appears: ```An OpenAPI operation has already been added for ...```

The reason is that currently only method and path are taken into account when adding a new routes, while `format` should be considered as well.

(Thanks a lot for a nice crate!)