Before this change attempting to return an error from core/command
failed with a 500 error and a message about unmarshable types.
This is because it was attempting to marshal the input parameters
which get _response added to them which contains an unmarshalable
field.
This was fixed by using the original parameters in the error response
rather than the one modified during the error handling.
This also adds end to end tests for the streaming facilities as used
in core/command.
a7689d7023