dont rely on changed property for validity class

This is the same as #13 but I opened a new PR because even when calling `handleInputChanged` in `beforeSubmit` it would still hide the `validityClass` in cases where the input was "unchanged".

It seems like no matter what, once `shouldValidate` is true (like it gets set inside `beforeSubmit`) we should display the `validityClass`.

This PR just removes relying on `!this.changed` inside the `validityClass` derived property.