fix issue when it is not possible to use package with 'cache: false' opt...

When using `cache: false` option in initial `options` object, then it is freezed on each find query because actual contents of arguments in `proto.exec` function are replaced in the begin of the function:

    if (typeof op === 'function') {
             callback = op;
             op = null;
    }

So when it tries to use default mongoose behaviour, mongoose doesn't receive valid callback and freezes.