fix($compile): set attribute value even if ngAttr* contains no interpolation

[gkalpak]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Bug fix

What is the current behavior? (You can also link to an open issue here)
Previoulsy, when the value of an ngAttrXyz attribute did not contain any interpolation, then the xyz attribute was never set.
See #15133.

Does this PR introduce a breaking change?
No.

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:
BTW, this commit adds a negligible overhead (since we have to set up a one-time watcher for
example), but it is justifiable for someone that is using ngAttrXyz (instead of xyz directly).

Fixes #15133.

[Narretz]

LGTM,
although I can't really see that with this change there's any possibility of adding an attribute interpolate directive that does not have an embedded expression. If you are using interpolation markers, then everything inside is treated as an expression, and only if you use interpolation markers a interpolation directive is added. That means you technically don't have to require mustHaveExpression, because it'll always have one, no?

[gkalpak]

Not really (if I understand correctly what you are asking 😁).

We call addAttrInterpolateDirective(...) for every attribute and it is addAttrInterpolateDirective's job to find out whether there is an interpolation and an actual directive needs to be added.

In order to do this, it passed the string value to $interpolate and checks whether interpolate return something. (With mustHaveExpression true, $interpolate will only return "something" - i.e. an interpolation function - if there is something to interpolate.)

In case of constant values in ngAttrXyz, $interpolate would return undefined (because of mustHaveExpression) and no directive would be added, which means the xyz attribute would not be set.

With this change though, since mustHaveExpression will be false for ngAttrXyz, $interpolate will return something (even if there is not expression in the string value) and a directive will be set up, which will in turn set the xyz property.

(And because $interpolate will return a special interpolation function it has for constants, the created watcher will be unregistered after the first time it is triggered, so there is no extra cost - except for the initial directive+watcher setup cost, of course.)