making update statement return empty result set when criteria would affect 0 rows

this change is to fix #70.

The corresponding test for the waterline-adapter-tests project would go in lib/collection/update.test.js somewhere inside the describe('attributes') block (so that it has some test data to run against)

```js
describe('when querying with params that will not match any records', function () {
  it('should not return an error and return an empty result set', function (done) {
    User.update('somenonexistantid', { type: 'invalid update' }, function (err, users) {
      assert(!err);
      assert(users.length === 0);
      done();
    });
  });
});
```