EAGLESIX-2696 First pass at test cases and minor code changes for ConstantExpression.
EAGLESIX-2651: Const: more tests, fix serialize bug
Merge pull request #30 from RiveraGroup/feature/mongo_2.6.5_expressions_Const

First pass at test cases and minor code changes for ConstantExpression.
Merge branch 'feature/mongo_2.6.5_expressions' into feature/mongo_2.6.5_expressions_ifNull_Cond_Not