Do not reset $viewValue back to the uninitialized widget value.

If currentVal != ngModel.$viewValue, and we reset widget.value() to be
the new ngModel.$viewValue, then it never makes sense to set
ngModel.$viewValue back to the old value.

Fixes issue #244.
Merge pull request #245 from astral303/issue-244

Do not reset $viewValue back to the uninitialized widget value.