fix for #291

[AnnMarieW]

fixes #291
fixes #260
fixes #304

This PR ensures that the values in a dropdown are set correctly when the data prop is updated in a callback.

Checked that it worked with all valid formats for the data prop:

1.  an array of strings 
2.  an array of dicts with label and value properties.
3.  an array of dict with group and items as keys where items are one of the previous two types.

[snehilvj]

Thanks a lot Ann!