title: expect{ subject }.to change(...).by(1) doesn't work
body: <p>This test passes:</p>
  
  <pre><code>it 'test' do
    old = parenthood.student.balance
    Payment.money_transfer_to_child current_user: parenthood.user,
                                    student_id: parenthood.student.id,
                                    amount: '1',
                                    comment: 'some comment'
    expect(parenthood.student.reload.balance).to eq(old+1)
  end
  </code></pre>
  
  <p>changing it to the following form makes it not pass ("was changed by 0"):</p>
  
  <pre><code>subject { Payment.money_transfer_to_child current_user: parenthood.user,
                                                student_id: parenthood.student.id,
                                                amount: '1',
                                                comment: 'some comment' }
  it { expect{ subject }.to change(parenthood.student.reload, :balance).by(1) }
  </code></pre>
  
  <p>What am I doing wrong?</p>