Add tests for more complex array-handling behaviors.

Added failing tests for proper handling of arrays in documents being queried and the positional projection operator (http://docs.mongodb.org/manual/reference/operator/projection/positional/#proj._S_)

Also adds a passing test to ensure that requesting a field requires that all returned documents have that field.

Fixes a type and removes unnecessary imports.

I'm interested in making these tests pass, but it looks like it will require seriously reorganizing some of the code implementing Collection.find().