Change mongodb near query so that a nil radius omits the maxDistance instead of setting to 0.0

Currently, when a nil radius is passed to the near scope, maxDistance is set to 0.0 which will not return any results. Since maxDistance is optional (http://docs.mongodb.org/manual/reference/operator/query/near/#op._S_near), I think it would be nice to not force maxDistance to be set. This pull requests checks to see if radius is not nil before setting the maxDistance operator.