fix(input.min/max) re-validate input[type=number] when min/max changes

[quazzie]

Value was not re-evaluated when min/max changed.

[mary-poppins]

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[njs50]

If this isn't going to be merged soon it might be worthwhile updating the docs to note that the expected behaviour doesn't work?

[tbosch]

We are working on refactoring the forms validation, see #6928.

[njs50]

Nice!

If it does slip from 1.3 it would be nice to have this PR in the meantime.

[greglockwood]

It appears that this PR does not address a related issue where if the value for the min/max attribute is "" (empty string) when the input is compiled, but changes to a valid number later, then the validator functions are never added correctly, even when the attribute value changes.

This is because of the if (attr.min) / if (attr.max) conditions that will evaluate as false if the attribute value is initially blank.

This is most easily observed if you try to use an interpolated expression as the value for the min/max attributes where the expression initially evaluates to undefined but in a subsequent cycle evaluates to a number:
e.g. <input type="number" ng-model="some.property" min="{{ somePropMetaData.min }}"> where $scope.somePropMetaData gets set after the input has been compiled.

There is a workaround where you ensure that a non-empty value is initially set for the min/max attribute so the if condition passes:
<input type="number" ng-model="some.property" min="{{ somePropMetaData ? somePropMetaData.min : 0}}">
but it is not clear from the documentation that such a workaround is required and it requires close examination of the source code to notice that it is.

[Narretz]

Last remnants of this issue are discussed here: #2404