When testing:

```ruby
post articles_path, params: { article: { title: 'Ahoy!' } }, as: :json
```

It's common to want to make assertions on the response body. Perhaps the
server responded with JSON, so you write `JSON.parse(response.body)`.
But that gets tedious real quick.

Instead add `parsed_body` which will automatically parse the reponse
body as what the last request was encoded `as`.