Feature/mongo 2.6.5 accumulators Avg

Updated Avg Accumulator and tests from 2.6.5.

Here are the original source code files for reference:
* https://github.com/mongodb/mongo/blob/r2.6.5/src/mongo/dbtests/accumulatortests.cpp#L61
* https://github.com/mongodb/mongo/blob/r2.6.5/src/mongo/db/pipeline/accumulator_avg.cpp
No need. It is handled on the `processInternal()` side.