Select2 widgets should take 100% parent width

[ioleo]

For some reason, when adding multiple select2 items (for example, in symfony2admingenerator/generator-bundle's filter group) they don't always take up 100% of parent space.

[ioleo]

When resizing the browser window, the select2 boxes keep their fixed width, while they should shrink to fit 100% of parent box.