Saving a Component with Default TRUE Should Un-set Default for Other Instances

CiviMail allows multiple instances of civicrm_mailing_component record with the same "component_type". Currrently, if you add another instance of a component (/civicrm/mailing/component?action=add&reset=1) - and set Default = TRUE, you wind up with two records of the same type, both with is_default = 1.

Create and Update component should set is_default = 0 for any existing components with the same component type if the current record has is_default = 1.