Removes duplicate func and adds tests.

test/services/users.js

@@ -178,6 +178,25 @@ describe('services.UsersService', () => {
     });
   });
 
+  describe('#setDisplayName', () => {
+    it('should set the display name to a new unique one', () => {
+      return UsersService
+        .setDisplayName(mockUsers[0].id, 'maria')
+        .then(() => UsersService.findById(mockUsers[0].id))
+        .then((user) => {
+          expect(user).to.have.property('displayName', 'maria');
+        });
+    });
+
+    it('should return an error when the displayName is not unique', () => {
+      return UsersService
+        .setDisplayName(mockUsers[0].id, 'marvel')
+        .catch((error) => {
+          expect(error).to.not.be.null;
+        });
+    });
+  });
+
   describe('#ban', () => {
     it('should set the status to banned', () => {
       return UsersService