AddressItem should override FieldItemBase::mainPropertyName()

FieldItemBase defines "value" as the default main property name for a field item. AddressItem field types do not seem to have a "value" property defined, and this causes errors in other areas of Drupal. Namely, when other modules such as Rules are attempting to determine the type of data of this field, they are unable to figure it out because "value" doesn't have a valid data type for address fields.

I'd like to submit a patch for this, however I'm not entirely sure what the correct mainPropertyName should be for an address field. Any insight into this?