MongoDB - loopback.getCurrentContext() return null

loopback.getCurrentContext() return null after MongoDB call returns.

wrap up @Ichenay's patch https://github.com/lchenay/loopback/commit/f075c79 (see this comment for more details: https://github.com/strongloop/loopback/pull/337#issuecomment-61680577)

Fix #809
Please move the test to `test/loopback.test.js` and shorten the describe label to `loopback.getCurrentContext`.