fix i18n keys for duration

app/contracts/work_packages/base_contract.rb

@@ -318,7 +318,7 @@ module WorkPackages
     end
 
     def validate_duration_integer
-      errors.add :duration, :only_integer if model.duration_before_type_cast != model.duration
+      errors.add :duration, :not_an_integer if model.duration_before_type_cast != model.duration
     end
 
     def validate_duration_matches_dates

config/locales/en.yml

@@ -578,6 +578,7 @@ en:
         begin_deletion: "Begin of the deletion"
         children: "Subelements"
         done_ratio: "Progress (%)"
+        duration: "Duration"
         end_insertion: "End of the insertion"
         end_deletion: "End of the deletion"
         version: "Version"

spec/contracts/work_packages/base_contract_spec.rb

@@ -498,7 +498,7 @@ describe WorkPackages::BaseContract do
         work_package.duration = 4.5
       end
 
-      it_behaves_like 'contract is invalid', duration: :only_integer
+      it_behaves_like 'contract is invalid', duration: :not_an_integer
     end
 
     context 'when setting the duration to a negative value' do