Cannot encode annotations types with nested Maps that differ from the annotation key type

The MapType encoder has a fixed key type setting used to enforce the types of keys for things like MessageAnnoations, DeliveryAnnotations that define Symbol type keys or ApplicationProperties that use String keys.  The current code uses that fixed type in error when encoding nested Maps within which causes the encode to fail if the nested map has a differing key type.  The MapType encoder needs to ignore the fixed key type setting when encoding values of the map entries.