Fix return correct 409 conflit error code

When registering or updating a user with a duplicate, albeit differently cased
username a 400 error would fire, as opposed to the preferred 409

lib/api/users.rb

@@ -164,11 +164,11 @@ module API
           present user, with: Entities::UserPublic, current_user: current_user
         else
           conflict!('Email has already been taken') if User
-              .where(email: user.email)
+              .iwhere(email: user.email)
               .count > 0
 
           conflict!('Username has already been taken') if User
-              .where(username: user.username)
+              .iwhere(username: user.username)
               .count > 0
 
           render_validation_error!(user)
@@ -196,11 +196,11 @@ module API
         not_found!('User') unless user
 
         conflict!('Email has already been taken') if params[:email] &&
-            User.where(email: params[:email])
+            User.iwhere(email: params[:email])
                 .where.not(id: user.id).count > 0
 
         conflict!('Username has already been taken') if params[:username] &&
-            User.where(username: params[:username])
+            User.iwhere(username: params[:username])
                 .where.not(id: user.id).count > 0
 
         user_params = declared_params(include_missing: false)

spec/requests/api/users_spec.rb

@@ -551,6 +551,18 @@ describe API::Users do
         expect(json_response['message']).to eq('Username has already been taken')
       end
 
+      it 'returns 409 conflict error if same username exists (case insensitive)' do
+        expect do
+          post api('/users', admin),
+               name: 'foo',
+               email: 'foo@example.com',
+               password: 'password',
+               username: 'TEST'
+        end.to change { User.count }.by(0)
+        expect(response).to have_gitlab_http_status(409)
+        expect(json_response['message']).to eq('Username has already been taken')
+      end
+
       it 'creates user with new identity' do
         post api("/users", admin), attributes_for(:user, provider: 'github', extern_uid: '67890')