Update event-handler-spec-example.html to match the current IDL spec.

https://github.com/heycam/webidl/commit/a2d5851e31447bea2b10c08d5aa8f00cd5276925
Add another test to event-handler-spec-example.html for the case without the initial assignment to button.onclick.
Merge pull request #506 from Ms2ger/update-event-handler-test

Update event-handler-spec-example.html to match the current IDL spec; r=bz