Add "valueAllowUnset" option that sets the value and options bindings so that a value that is not in the list of options is allowed and is displayed accordingly with no item selected.
Update code to set the value to the current UI selection when the value is rejected. Update test to check this.
Reinstate behavior when clearing value for size > 1 select.
In selectExtensions, use === instead of == where possible.
Merge pull request #647 from knockout/647-allow-unselected-value

Support select value not in options list, optionally