Change style of `upon_receiving` messages
Change style of `upon_receiving` messages

these are intended as human readable descriptions of the operation.
Tweak them to fit in better with pact's style and the way that the
output is presented, e.g.:

```
  Given both content stores and url-arbiter empty
    a request to create a content item
      with PUT /content/test-content-item
        returns a response which
          has status code 200
          has a matching body
          includes headers
            "Content-Type" with value "application/json; charset=utf-8"
```

alphagov/gds-api-adapters