In User Defined Output plugin framework, handleResponseData called twice per request

[dubious90]

It is possible that it is being called on both the request path and the response path. More investigation is required here.

When called in integration tests, it is always first empty data followed by the expected response.

[dubious90]

I've confirmed that we are not calling this on the request path, only the response path. New theory is that this is just envoy sending response data in chunks, though more investigation is required as to why there is an empty data buffer sent ahead of each actual response.