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.

NOTES:
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- Using java-driver 2.12.x when connecting to an 2.8.0.rc-0 instance is likely to cause trouble with authentication. This is the intended behavior. 

- 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 java-driver 2.13.0-rc0 works perfectly fine with a 2.6.x Server instance.

- We deprecated Index.Duplicates#DROP since it has been removed in MongoDB 2.8