EAGLESIX-2695 bugfix. restored the check that ensures two arguments are passed in. Added rudimentary test cases.
EAGLESIX-2695 Added the check for the number of arguments. Added rudimentary tests.
Merge branch 'bugfix/mongo_2.6.5_expressions_Let' into feature/mongo_2.6.5_expressions_Let

Conflicts:
	test/lib/pipeline/expressions/LetExpression_test.js