Ensure that issues and merge requests are found

doc/api/merge_requests.md

@@ -680,6 +680,7 @@ Example response:
   "subscribed": true
 }
 ```
+
 ## Unsubscribe from a merge request
 
 Unsubscribes the authenticated user from a merge request to not receive

lib/api/issues.rb

@@ -239,8 +239,8 @@ module API
       #  issue_id (required) - The ID of a project issue
       # Example Request:
       #   POST /projects/:id/issues/:issue_id/subscription
-      post ":id/issues/:issue_id/subscription" do
-        issue = user_project.issues.find_by(id: params[:issue_id])
+      post ':id/issues/:issue_id/subscription' do
+        issue = user_project.issues.find(params[:issue_id])
 
         if issue.subscribed?(current_user)
           not_modified!
@@ -257,8 +257,8 @@ module API
       #  issue_id (required) - The ID of a project issue
       # Example Request:
       #   DELETE /projects/:id/issues/:issue_id/subscription
-      delete ":id/issues/:issue_id/subscription" do
-        issue = user_project.issues.find_by(id: params[:issue_id])
+      delete ':id/issues/:issue_id/subscription' do
+        issue = user_project.issues.find(params[:issue_id])
 
         if issue.subscribed?(current_user)
           issue.unsubscribe(current_user)

spec/requests/api/issues_spec.rb

@@ -594,20 +594,20 @@ describe API::API, api: true  do
 
   describe 'DELETE :id/issues/:issue_id/subscription' do
     it 'unsubscribes from an issue' do
-      post api("/projects/#{project.id}/issues/#{issue.id}/subscription", user)
+      delete api("/projects/#{project.id}/issues/#{issue.id}/subscription", user)
 
       expect(response.status).to eq(200)
       expect(json_response['subscribed']).to eq(false)
     end
 
     it 'returns 304 if not subscribed' do
-      post api("/projects/#{project.id}/issues/#{issue.id}/subscription", user2)
+      delete api("/projects/#{project.id}/issues/#{issue.id}/subscription", user2)
 
       expect(response.status).to eq(304)
     end
 
     it 'returns 404 if the issue is not found' do
-      post api("/projects/#{project.id}/issues/123/subscription", user)
+      delete api("/projects/#{project.id}/issues/123/subscription", user)
 
       expect(response.status).to eq(404)
     end

spec/requests/api/merge_requests_spec.rb

@@ -529,6 +529,12 @@ describe API::API, api: true  do
 
       expect(response.status).to eq(304)
     end
+
+    it 'returns 404 if the merge request is not found' do
+      post api("/projects/#{project.id}/merge_requests/123/subscription", user)
+
+      expect(response.status).to eq(404)
+    end
   end
 
   describe 'DELETE :id/merge_requests/:merge_request_id/subscription' do
@@ -544,6 +550,12 @@ describe API::API, api: true  do
 
       expect(response.status).to eq(304)
     end
+
+    it 'returns 404 if the merge request is not found' do
+      post api("/projects/#{project.id}/merge_requests/123/subscription", user)
+
+      expect(response.status).to eq(404)
+    end
   end
 
   def mr_with_later_created_and_updated_at_time