add explicit unset for the image routes

the initial motivation was to fix up the profile avatar rule which
had a mix of json or multipart. this style does not work well with
express 5

src/routes/test/profile-test.js

@@ -373,6 +373,45 @@ describe('Profile API', function () {
         });
     });
 
+    describe('background', function () {
+        it('no default', async function () {
+            const response = await superagent.get(`${serverUrl}/api/v1/profile/background_image`)
+                .query({ access_token: user.token })
+                .ok(() => true);
+            expect(response.status).to.be(404);
+        });
+
+        it('can set custom background', async function () {
+            const response = await superagent.post(`${serverUrl}/api/v1/profile/background_image`)
+                .query({ access_token: user.token })
+                .attach('backgroundImage', './logo.png');
+
+            expect(response.status).to.be(200);
+        });
+
+        it('did set custom background', async function () {
+            const response2 = await superagent.get(`${serverUrl}/api/v1/profile/background_image`)
+                .query({ access_token: user.token })
+                .ok(() => true);
+
+            const customAvatarSize = require('fs').readFileSync('./logo.png').length;
+            expect(parseInt(response2.headers['content-length'])).to.equal(customAvatarSize);
+            expect(response2.status).to.equal(200);
+        });
+
+        it('can unset background', async function () {
+            const response = await superagent.del(`${serverUrl}/api/v1/profile/background_image`)
+                .query({ access_token: user.token });
+
+            expect(response.status).to.be(200);
+
+            const response2 = await superagent.get(`${serverUrl}/api/v1/profile/background_image`)
+                .query({ access_token: user.token })
+                .ok(() => true);
+            expect(response2.status).to.be(404);
+        });
+    });
+
     describe('language', function () {
         it('fails to set unknown language', async function () {
             const response = await superagent.post(`${serverUrl}/api/v1/profile/language`)