A button would appear disabled, if button.disabled==False

If, when the template is rendered, button.disabled is False, then a disabled="" attribute is rendered, which is enough to disable the submit button.
To avoid this, I am using this code:

tal:attributes="disabled button.disabled or None"

I added a second commit by accident; it is a new feature which should be in a different pull request. If you want me to, I can redo the whole thing as 2 separate pull requests... Just let me know.