Fix RobinEngineIntegrationTest - missed to explicitly create the index
Make the get source REST API source fetching query parameters consistent with other API

 The get source rest endpoint now uses _source_* parameters.
 Also it will now throw a validation error if you turn off source fetching.

Closes #3886
fix naming in function_score

    - "boost" should be "boost_factor"
    - "mult" should be "multiply"

closes #3872