DATAMONGO-1092 - Ensure compatibility with MongoDB 2.8.0.rc0 and java driver 2.13.0-rc0.

We updated `GroupByResults` to allow working with changed data types returned for _count_ and _keys_ and fixed assertion on error message for duplicate keys. 

`MongoTemplateTests#testEnsureIndex` could not be fixed so far (see notes below for more info).

**NOTES:**
- Using java-driver `2.12.x` when connecting to a `2.8.0.rc-0` server instance is likely to cause trouble with authentication. This is the intended behavior.
- The java-driver `2.13.0-rc0` worked with a `2.6.x` server.
- `2.8.0-rc0` throws error when removing elements from a collection that does not yet exist, which is different to what `2.6.x` does.
- The _dropDups_ attribute is not returned for `getIndexes()` even when the index was defined with `dropDups:true`. Therfore `testEnsureIndex` test still fails.