batch processor: always prioritize tags from model

[RafalSkolasinski]

Seldon models always prioritise in their response tags generated by the model itself.
This was not the case for batch processor with raw inputs and batch_size > 1 (not released, still on master).

This is to fix it.

Existing batch processor tests still passes:

est_batch_processor.py::TestBatchWorker::test_batch_worker PASSED                                                                    [ 33%]
test_batch_processor.py::TestBatchWorker::test_batch_worker_raw_predict_ndarray PASSED                                                [ 66%]
test_batch_processor.py::TestBatchWorker::test_batch_worker_raw_predict_tensor PASSED                                                 [100%]

[RafalSkolasinski]

/test integration

[seldondev]

@RafalSkolasinski: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
integration	07d4bb1	link	/test integration

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[axsaucedo]

Seems failure is on rolling update tests which are flaky and code looks good, merging
/approve

[seldondev]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: axsaucedo

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