2dsphere explain reports "works" for nscanned & nscannedObjects

RE: https://github.com/mongodb/mongo/blob/r2.6.6/src/mongo/db/query/explain_plan.cpp#L266

When an explain is run on a $nearSphere query against a 2dsphere index we are incorrectly recording nscanned and nscannedObjects to be the "works" value rather than the actual # of index entries and documents evaluated.

This has been corrected for 2.8 as part of SERVER-9986 but not for 2.6. As SERVER-9986 is a broad ticket covering much more than this issue I am entering a separate ticket to address 2.6.