fix(select): normalize options without value attrs

In order to fix #6351 I'm submitting this changes based on @caitp comments.
Note that the [commented expectation](https://github.com/cironunes/angular.js/blob/faeca99fddf40a277489a2ce9d3783904768422f/test/ng/directive/selectSpec.js#L169) is returning ```[["? undefined:undefined ?"],"--select--","x","y"]``` is it expected?