test runner now serializes response data

This makes for a more robust test.  It will alert devs to serialization
errors in their tests.

This is useful b/c certain versions of BSON error on certain data
types (like `Date` and `DateTime`).  This means your tests can not
only validate the response data itself, but that it will also
serialize.

Closes #81.
Closes #82.

@jcredding ready for review.  We discussed taking this approach as it is less overhead and does less assuming on the part of the users, but still alerts devs to when their response data won't serialize.