Fix user API tests succeeding for the wrong reason

The requests were missing multiple required attributes, while testing for single attribute missing.
Added test for checking username as required attribute.

spec/requests/api/users_spec.rb

@@ -110,17 +110,22 @@ describe API::API, api: true  do
     end
 
     it 'should return 400 error if name not given' do
-      post api('/users', admin), email: 'test@example.com', password: 'pass1234'
+      post api('/users', admin), attributes_for(:user).except(:name)
       expect(response.status).to eq(400)
     end
 
     it 'should return 400 error if password not given' do
-      post api('/users', admin), email: 'test@example.com', name: 'test'
+      post api('/users', admin), attributes_for(:user).except(:password)
       expect(response.status).to eq(400)
     end
 
-    it "should return 400 error if email not given" do
-      post api('/users', admin), password: 'pass1234', name: 'test'
+    it 'should return 400 error if email not given' do
+      post api('/users', admin), attributes_for(:user).except(:email)
+      expect(response.status).to eq(400)
+    end
+
+    it 'should return 400 error if username not given' do
+      post api('/users', admin), attributes_for(:user).except(:username)
       expect(response.status).to eq(400)
     end