Suggest REST endpoint fix for POST/PUT

Fix an odd behavior whereby a user can create documents with id=_suggest
by issuing POST/PUT requests to
http://localhost:9200/{index}/{type}/_suggest.

This fix intercepts such requests and returns a 400 BAD_REQUEST
response.

Additionally, this commit includes an extension to the standard
integration test facilities which allows a subclass to use a RestClient
instance to issue REST requests.

Closes #5442