Work with projection with boolean value.

Before this change only numeric projections ({foo: 1} were accepted, while
boolean projection are valid ({foo: true}.

In particular Morphia (https://code.google.com/p/morphia/)
Query#retrievedFields creates a projection using booleans.
Merge pull request #84 from zauberlabs/projections-with-boolean-values

Work with projection with boolean value.