Merge branch 'fix-collection-serializer' into 'master'

add relation belongs_to owner

See merge request !350

app/serializers/collection_serializer.rb

@@ -13,7 +13,8 @@ class CollectionSerializer < ActiveModel::Serializer
     object.complained? current_user
   end
 
-  attributes :id, :name, :owner, :created_at, :updated_at, :description, :privacy, :score, :likes_count, :liked, :reviewed, :complained, :review_average, :thumbnail
+  attributes :id, :name, :created_at, :updated_at, :description, :privacy, :score, :likes_count, :liked, :reviewed, :complained, :review_average, :thumbnail
+  belongs_to :owner
   has_many :tags
   has_many :subjects
   has_many :educational_stages