Improve the test for change events when calling fill_in.

  - Assert that the event is triggered.
  - Assert that the value of the input is correct at the time the event
    is triggered.
  - Do not allow multiple events to be fired for a single change.
Specify :match => :one explicitly.
Merge pull request #1054 from TimMoore/improve-change-event-test

Improve the test for change events when calling fill_in.