Feature/mongo 2.6.5 accumulators First

Updated First accumulator and tests to match v2.6.5.

Here are the original source code files for reference:
* https://github.com/mongodb/mongo/blob/r2.6.5/src/mongo/db/pipeline/accumulator_first.cpp
* https://github.com/mongodb/mongo/blob/r2.6.5/src/mongo/dbtests/accumulatortests.cpp#L288