CollectingResponse inconsistency with actual response types

[porcellus]

In our guides we recommend people to implement sending custom responses in API overrides by calling sendJSONResponse on the response class and that the object returned by the override will be ignored. This doesn't work in the current implementation since later calls to sendJSONResponse (from a non-overrideable part of the SDK) will actually overwrite the response set in the override instead of simulating what other response types do, where the later calls to sendJSONResponse are ignored. This is causing issues in our in NextJS integration.