Allow users to configure APIs to not convert return values to JSON

[deliahu]

Description

Allow users to configure APIs to not convert return Predictor values to JSON; Predict() should then return a string which would be passed through unmodified to the response.

Motivation

Converting to JSON can be time consuming on large tensors; a faster approach may be to serialize the tensor using e.g. pickle.dumps() and then respond with the string.

Note that to use pickle.dumps(), the client must be in Python, but there still may be other valid/faster serialization tools other than json

[deliahu]

Closing in favor of #913