chips: editable chip removes item when the value is updated

[DeneroWatz]

Actual Behavior:

• What is the issue? *

Modifying a value actually deletes the value/chip unless you manually select the old value before typing in the new value.

On Chrome 57.0.2987.133(version shouldn't matter), go to https://material.angularjs.org/latest/demo/chips and then select an item under 'Make chips editable' field.

If you don't select the old value but clear it using Backspace or Delete keys then type 'Kiwi' in it, that element of the array seems to get deleted and the chip disappears when you press Enter.

• What is the expected behavior?
  It should not remove, but update the existing item.

CodePen (or steps to reproduce the issue): *
Reproduceable here - https://material.angularjs.org/latest/demo/chips

AngularJS Versions: *

• AngularJS Version: 1.5.5
• AngularJS Material Version: 1.1.3

Additional Information:

• Browser Type: * Chrome
• Browser Version: * 57.0.2987.133
• OS: * Windows 10
• Stack Traces: None

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