AMS: payload key should be type, not attr name

Fixes embedded object extraction when the attribute name is not
the plural form of the relationship's target type.
AMS: embedded objects should always be sideloaded

Allows for tree structures where the embedded type is the same
as the parent type.
AMS: tests: multiple embedded hasMany w/ same type
Merge pull request #1357 from tstirrat/issue/1356

ActiveModelSerializer: Handle embedded objects when attribute name is not plural of type