Fixed a bug where descending indexes were not being processed correctly in Fongo. Index keys are resolved by
piggy-backing on the FongoDBCollection.applyProjections mechanism, which considers fields in the projection
as excluded if their value is not greater than 0. This obviously conflicts with the fact that a value of -1
in the index object is perfectly legal. Best fix I could think of that still preserves the re-use of this method
was to set to the condition for inclusion to be based on if the projection field value is not equal to 0.