Don't change the datatype of values bound to form elements

See https://github.com/ractivejs/ractive/issues/488

@Rich-Harris says "so basically I think we should keep things as are for the moment, and revisit it once 0.4.0 is out the door." So I don't expect this to get merged immediately. I am providing this PR as a reminder that this issue needs to be revisited at some point.