Add form "acceptCharset" to HTMLDOMPropertyConfig

What the title says.
http://www.w3.org/TR/html5/forms.html#attr-form-accept-charset
```JS
f = document.createElement('form');
f.acceptCharset = 'a';
console.log(f.getAttribute('accept-charset'));
f.setAttribute('accept-charset', 'b');
console.log(f.acceptCharset);
```

Prints A & B correctly for all browsers I have access to.