Fix so disabled controls are totally ignored from built up object model

[mcintyre321]

...put values, so shouldn't be included in results

#36

[mcintyre321]

Given a form containing

<input type="hidden" name="x" value="hello" />
<input type="hidden" name="x" value="hello" disabled="disabled" />

form2js will currently return a null value for x - disabled controls should be totally ignored and instead form2js should return {x:"hello"}

[lookfirst]

Nice fix. I checked this into my fork.

[maxatwork]

Disabled controls are intentionally excluded by default in the current DOM extractor, including disabled fieldset descendants unless getDisabled is enabled. That behavior is documented and covered in packages/dom/test/dom.test.ts, so I am closing this legacy PR as already addressed.