Fix belongsTo and belongToMany relations

belongsTo and belongToMany would resolve to an incorrect relation name
if it was a relation with a SQL model, due to MongoModel::belongsTo
being the caller instead of MongoModel::relation which makes foreignKey
become "belongs_to_id", the unit tests do not show this behavior because
foreignKey is set explicitly.

Backport Eloquent BelongsToMany::sync function signature

Add missing abstract methods from UserInterface in User test model

Fix #175 : embedsMany duplicates are identified by key