fix(select): don't support binding to select[multiple]

[IgorMinar]

changing the type of select box from single to multiple or the other way around
at runtime is currently not supported and the two-way binding does odd stuff
when such situation happens.

we might eventually support this, but for now we are just going to not allow
binding to select[multiple] to prevent people from relying on something that
doesn't work.

BREAKING CHANGE: binding to select[multiple] directly or via ngMultiple (ng-multiple)
directive is not supported. This feature never worked with two-way data-binding,
so it's not expected that anybody actually depends on it.

Closes #3230

[KJTsanaktsidis]

Can I ask, what problems exactly did my pull request in #3230 not fix? If they're not serious, I'm happy to fix them, this is something that we use.

[IgorMinar]

@KJTsanaktsidis how do you use ngMultiple? the binding system breaks down when you use data-binding for <select ng-multiple="isMultiple" ng-model="mySelectModel"> and change isMultiple at runtime. This has never worked, we just never noticed.

[IgorMinar]

by breaking down I mean that it's not clear what actually should happen - when multiple the ng-model is an array, for simple select the model is a reference to the value specified via option or ng-options. how do you transition this model in a way that makes sense?

if you want to think about it and work out all the quirks, then please open a new PR.

[IgorMinar]

the change in this PR has landed as d87fa00