fix(angular.equals): relax the comparison for undefined properties

in 5ae63fd3 the comparison was made consistent but strict, so that

angular.equals({}, {foo: undefined}) // always returns false

this turns out to cause issues for data that is being roundtripped via network
and serialized via JSON because JSON.stringify serializes {foo: undefined} as {}.

Since angular.equals() behaved like this before the 5ae63fd3 in 50% of the cases,
changing the behavior in this way should not introduce any significant issues.

Closes #1648