[Managed SDK] More unit test improvements

- Refactored MobileServiceTable tests to share a common base class and moved the JObject helpers there.
- Expanded the JObject helpers to include long types and objects that don't contain an ID at all.
- Plowing through the tests to improve readability and eliminate magic strings wherever possible. This will be an ongoing process.
Merge branch 'master' of https://github.com/advancedrei/azure-mobile-services