fix(datepicker): validate using ng-required attribute

[nivivive]

This is a solution for the the issue (#3862), where date is always required for datepicker, even when ng-required is set to false.

(todo: still need to add tests)

[nivivive]

@wesleycho, moving the discussion to this PR:

In response to your comments, I had some questions. Would the proper fix be to:

1. log the error: "Datepicker directive: "ng-model" value must be a Date object..." regardless of what ng-required is set as
2. remove ngModelCtrl.$setValidity('date', ...) from this.render() and move all the validation logic to ngModelCtrl.$validators.date

[wesleycho]

Correct on both counts

[nivivive]

does this look good?

[wesleycho]

Is this even necessary? Does this run if ngRequired is falsy?

[nivivive]

Yes, it works even if ngRequired is falsy. If ngRequired is falsy (i.e. anything other than ng-required=true), then the datepicker is not required. So, it considers an empty value valid in that case, but will check for date validity for any non-empty value.

[wesleycho]

I wasn't saying just working - I was wondering whether the validator even runs if !attrs.ngRequired. If it doesn't run, it wouldn't be necessary, but if it does, then this line is fine.

[nivivive]

The validator does run if !attrs.ngRequired

[wesleycho]

This LGTM, I will do a full check on Sunday and hopefully will merge it in shortly after.

[nivivive]

hey, is there any update on this?

[wesleycho]

Sorry, haven't had the chance to check this out yet, as it requires a little more setup/evaluation than the other stuff I have looked at so far.

I do intend on reviewing this thoroughly this week though.

[wesleycho]

Looks good here.