Renamed user to defaultAssignee in order to comply with the specification

lib/api/v3/categories/category_representer.rb

@@ -48,7 +48,7 @@ module API
           }
         end
 
-        link :user do
+        link :defaultAssignee do
           {
             href: api_v3_paths.user(represented.assigned_to.id),
             title: represented.assigned_to.name

spec/lib/api/v3/categories/category_representer_spec.rb

@@ -61,7 +61,7 @@ describe ::API::V3::Categories::CategoryRepresenter do
       it_behaves_like 'category has core values'
 
       it 'should not link to an assignee' do
-        expect(subject).to_not have_json_path('_links/user')
+        expect(subject).to_not have_json_path('_links/defaultAssignee')
       end
     end
 
@@ -72,10 +72,10 @@ describe ::API::V3::Categories::CategoryRepresenter do
       it_behaves_like 'category has core values'
 
       it 'should link to its default assignee' do
-        expect(subject).to have_json_path('_links/user/href')
+        expect(subject).to have_json_path('_links/defaultAssignee/href')
       end
       it 'should display the name of its default assignee' do
-        expect(subject).to have_json_path('_links/user/title')
+        expect(subject).to have_json_path('_links/defaultAssignee/title')
       end
     end
   end