rough try at addressing html5 types

[tommasop]

I'm trying to use a form input with type number.

This small patch solves the issue.

This is my first pull request so I don't know if this is a rough implementation that breaks something else.

My implementation is working fine with standard text and number inputs (defined through html_attrs: {type: 'number'} )

[thiago-negri]

I guess this can solve my problem, #559

I'll give it a try later