Make response handling more consistent and "safe"

[tonyheupel]

The RavenDB API doesn't seem to always send back JSON as it's response, so we need to:

1. Watch the return codes
2. Check the content-type of the response before trying a JSON.parse

This will take some playing with curl and the API or putting in a bunch of debug logging to see what comes back in various scenarios.

Please START with tests on this, tchype (that's me berating myself for not having better coverage).