Formatters for built-in validators should not return `undefined` for invalid values.

[niallsmart]

The $formatter function for all the built-in validators (email, minLength, etc) returns undefined if the model value is invalid. As a result:

• the model remains in an invalid state
• the corresponding ngForm and ngModel controllers have $invalid === true
• a validation error message is displayed to the user (depending on the application)
• the form field is totally empty, so the user is left scratching their head as to the cause of the problem 😕

The validators should return the invalid value "as-is" and allow the user to see and correct it. I am happy to work up a patch if the Angular team agrees with this suggested behavior.

Demo page here: http://plnkr.co/EneUPt

[jbdeboer]

Dupe of #2841 (lower issue numbers win)