Clear persisted value when default value is used

Otherwise, the last-used non-default value remains persisted for next
time. This results in surprising behavior where setting a field back to
the default works, but then does not persist the next time you run.

This commit is dedicated to Klim Kolyvanov.

Author: ctrueden

src/main/java/org/scijava/module/DefaultModuleService.java

@@ -294,6 +294,7 @@ public <T> void save(final ModuleItem<T> item, final T value) {
 			// NB: Do not persist the value if it is the default.
 			// This is nice if the default value might change later,
 			// such as when iteratively developing a script.
+			prefService.remove(prefClass(item), prefKey(item));
 			return;
 		}