Ignore disabled form inputs

[kklemon]

Disabled input elements are not in included in the emerging request in general according to 1 and 2 which doesn't seems to be recognized by lxml yet.

These changes do not really implements the standard perfectly which includes also rules for which the disabled attribute would be invalid, but it's actually the way most browsers are handling it by disabling an input just when the disabled attribute is set at all independently of the actual value.

[scoder]

Thanks. Please provide a test.

[kklemon]

Done.

It seems like for XHTML the corresponding attribute would have to be exactly disabled="disabled" as XHTML does not support minimized attributes. I'm not sure if lxml does really distinguish here, but I noticed that it maybe could make sense to recognize this in the xhtml_to_html() function in lxml.html by unminimizing HTML attributes when converting to XHTML.

[scoder]

I think your idea of converting these back and forth in xhtml_to_html() and html_to_xhtml() is a good one. Could you code up a pull request for that, too?