fix: properly handle input type='button'

This is first & fast attempt at fixing #193, but I wouldn't be surprised to see similar issues given the number of different input types (https://developer.mozilla.org/en/docs/Web/HTML/Element/Input). If I'm not mistaken we should have the same logic for submit buttons, at least. 

Anyway, opening a PR to see if tests pass on every browser and to kick off discussion on the design of inputs handling.