validateConfirmation does not revalidate if related property changes

## Version
1.2.8

## Test Case
https://ember-twiddle.com/8687b828162c979dceef152ba69d1be8?openFiles=controllers.application.js%2C

## Steps to reproduce
Enter missing "t" in "Password" input field.

## Expected Behavior
Password confirmation should be valid after both values are the same.

## Actual Behavior
It's not revalidated until password confirmation field changes.

## Other bugs in validateConfirmation
* #137, which is partly addressed by #138