Batched predictions: Segmentation fault and JSON parsing errors

[Corey-Zumar]

Steps to reproduce:

1. Start Clipper and register an appliction

2. (optional). Run the no-op container locally, register a model corresponding to the container, and link the model to the application

3. Send a batched input of type "doubles" to the application.

4. Observe that the query frontend crashes or emits the following log message: Unexpected error: JSON schema invalid. If this is not the case, repeat step 3 several times.

Note: This crash is related to the construction of the JSON response object with RapidJSON