Fixing reset method

[vicphase]

The iron-input elements inside the iron-form don't clear their value when the reset method is called.

Expected outcome

The iron-input elements should reset their value when the reset method of the iron-form is called.

Actual outcome

The iron-input elements keep their value when the reset method of the iron-form is called.

Steps to reproduce

1. Put an iron-form element on the page.
2. Put a iron-input element inside the iron-form element.
3. Type a value in the iron-input element
4. Call the reset method of the iron-form