fix($compile): bind ng-attr-* even if unbound attribute follows ng-attr-*

Previously, <element ng-attr-foo="{{binding}}" foo="bar"></element>'s "foo" attribute would always
equal "bar", because the bound version was overwritten. This CL corrects this behaviour and ensures
that the ordering of attributes does not have an effect on whether or not ng-attr-bound attributes
do their work.

Closes #7739