When creating a Content Block Definition of type "Dropdown from
UDT/simple_plugin" the choice from the "User Defined Tag" dropdown is not
stored.

To recreate:
1) Create a new definition
2) choose "Dropdown from UDT..." as the type
3) choose a UDT other than the first option
4) submit
5) reopen and see choice has changed to the first option

Additionally, this field doesn't show up in the content editor when added to the
page template.