This fixes the issue that caused two attr interpolation observers
to be registered for the same attribute as a result of isolate
scope definition with attr (@) property for this attribute.
Duplicate observers would then fight with each other updating the
model.
The issue occured only when this directive was used in a repeater
because that's when we clone the template node which caused the
two observers to point to two different sets of $attr instances.
Closes#1166, #836