No handling for HTML element extension

[cpboyd]

An example of this is iron-form, which extends the default form element.

The generated IronForm widget doesn't function properly.
It specifies an <iron-form> tag instead of <form is="iron-form">

Moreover, the get/setAction and get/setMethod are not inherited.

[manolo]

Yeah, that is something not considered because we expect all custom elements being new tags.
Will discuss with the team and consider this in a new version.