Amend entry when we already have one for today

If we already have an entry for today, then amend that entry instead of creating a new one.
I usually prefer to test things like this with a spy, rather than a mock. This lets the test continue to follow the 4 normal phases.

Here, I'd probably do 

```ruby
allow(AmendedEntry).to receive(:create!)

# process

expect(AmendedEntry).to have_received(:create!).with(...)
```