Button looses icon when loading state is changed

UiBinder XML:

    <b:Button ui:field='save' icon='SAVE' text='Save' loadingText='Saving...' completeText='Saved'/>

Java:

    saveButton.state().loading();
    ...
    saveButton.state().complete();

Icon is gone afterwards. Only new loading state text is displayed.
I think this happens because in [Bootstrap JavaScript](http://twitter.github.io/bootstrap/javascript.html#buttons)

    $(element).button("loading")

replaces the whole contents of the button with the loading state text.

I've tried to fix this by calling `setIconPosition()` in `Button.LoadingStateBehavior#setLoadingBehavior()` but got an `UmbrellaException`?!