fix(input): don't dirty model when input event triggered due to placeholder change

Certain versions of IE inexplicably trigger an input event in response to a
placeholder being set.

It is not possible to sniff for this behaviour nicely as the event is not
triggered if the element is not attached to the document, and the event
triggers asynchronously so it is not possible to accomplish this without
deferring DOM compilation and slowing down load times.

This change will possibly cause difficulties with HTML5 Constraint Validation.

In this initial commit, I'm just trying to get tests to fail on IE so I can
verify the issue on CI.