Steps to reproduce:
1. create a collection and a reverse index
2. insert two records into a collection
3. db.collection.validate(

) will report that the index has 0 keys in "keysPerIndex" field, while the expected number would be 2

I suspect the problem is here: https://github.com/mongodb/mongo/blob/v3.0/src/mongo/db/storage/wiredtiger/wiredtiger_index.cpp#L288

We're locating minKey, although minKey is actually at the end for the reverse index.

The same problem exists for RocksDB because I copied that part of the code