fix(select): handle model updates when options are manipulated

These rules follow ngOptions behavior:

- when an option that is currently selected, is removed or its value changes, the model
is set to null.
- when an an option is added or its value changes to match the currently selected model,
this option is selected.
- when an option is disabled, the model is set to null.
- when the model value changes to a value that matches a disabled option,
this option is selected (analogue to ngOptions)