Split multiple widget values when using ajax.

The recent fixes* to the multiple widget did not take into account that they use hidden input fields, and therefore the form value will be a string separated by commas.

This commit alters the behaviour so a list is only returned if ajax is not used, if ajax is used it will return a string which is split in the clean method.

*The fixes were in  commits 784cdab959e1401038a203ba921c54cd4abc406d & b4c633ec50d10c64058a74355552dfc93405e790