Fixed incorrect variable usage in MongoDbResourceManager

[careys7]

Updated incorrect $this->resource variable usages to $this->resources in MongoDbResourceManager.

[Ocramius]

Tests missing. Specifically: the fact that this didn't affect the build status means that we either:

• simply don't test it
• had unknown memory leaks
• it's dead code, and not relevant at all

[careys7]

Updated with tests.

Tests ensure that the instance of MongoCollection held for a given resource id does not persist following method calls to:

• MongoDbResourceManager::setServer()
• MongoDbResourceManager::setCollection()
• MongoDbResourceManager::setConnectionOptions()
• MongoDbResourceManager::setDriverOptions()
• MongoDbResourceManager::setDatabase()

[marc-mabe]

@careys7 Thanks for the fix! I have modified you test(s) to get batter coverage.

[careys7]

Your tests look better 👍