Editable FBox properties are not saved

FBox properties in the editor are not being saved.  This appears to be a regression caused by CL# 21566516 that was missed due to there being no editable box types in the Epic codebase.

• Load up QAGame.
• Add the following property to AQATestActor:

UPROPERTY(EditAnywhere, Category="Box")
FBox TestBox = {};

• Compile and run the editor.
• Drop an instance of QA Test Actor into the world.
• Select the instance and edit the fields of TestBox to some non-default value in the details panel.
• Save the map.
• Quit and re-load the editor.
• Load the saved map and observe the state of TestBox.

Expected: edits were saved and reloaded.

Actual: edits have reverted back to the default state.