Merge branch 'issuable-sidebar-dropdown-deselect-fix' into 'master'

Fixed issue with de-selecting dropdown option in issue sidebar

## What does this MR do?

When de-selecting either an assignee or milestone, we try to send `undefined` which jQuery removes from the request causing a routing error. Instead this MR sends `null` which jQuery still sends and then correctly removes either the milestone or assignee. 

## What are the relevant issue numbers?

Closes #18641 

## Screenshots (if relevant)

![assignee](/uploads/6eaca416fb6e31eabf4c038967160c07/assignee.gif)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- [ ] Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4671

app/assets/javascripts/milestone_select.js.coffee

@@ -116,7 +116,7 @@ class @MilestoneSelect
               .val()
             data = {}
             data[abilityName] = {}
-            data[abilityName].milestone_id = selected
+            data[abilityName].milestone_id = if selected? then selected else null
             $loading
               .fadeIn()
             $dropdown.trigger('loading.gl.dropdown')

app/assets/javascripts/users_select.js.coffee

@@ -31,7 +31,7 @@ class @UsersSelect
       assignTo = (selected) ->
         data = {}
         data[abilityName] = {}
-        data[abilityName].assignee_id = selected
+        data[abilityName].assignee_id = if selected? then selected else null
         $loading
           .fadeIn()
         $dropdown.trigger('loading.gl.dropdown')

spec/features/issues_spec.rb

@@ -396,6 +396,27 @@ describe 'Issues', feature: true do
           expect(page).to have_content @user.name
         end
       end
+
+      it 'allows user to unselect themselves', js: true do
+        issue2 = create(:issue, project: project, author: @user)
+        visit namespace_project_issue_path(project.namespace, project, issue2)
+
+        page.within '.assignee' do
+          click_link 'Edit'
+          click_link @user.name
+
+          page.within '.value' do
+            expect(page).to have_content @user.name
+          end
+
+          click_link 'Edit'
+          click_link @user.name
+
+          page.within '.value' do
+            expect(page).to have_content "No assignee"
+          end
+        end
+      end
     end
 
     context 'by unauthorized user' do
@@ -440,6 +461,26 @@ describe 'Issues', feature: true do
 
         expect(issue.reload.milestone).to be_nil
       end
+
+      it 'allows user to de-select milestone', js: true do
+        visit namespace_project_issue_path(project.namespace, project, issue)
+
+        page.within('.milestone') do
+          click_link 'Edit'
+          click_link milestone.title
+
+          page.within '.value' do
+            expect(page).to have_content milestone.title
+          end
+
+          click_link 'Edit'
+          click_link milestone.title
+
+          page.within '.value' do
+            expect(page).to have_content 'None'
+          end
+        end
+      end
     end
 
     context 'by unauthorized user' do