Checkbox command generates invalid HTML

[wwalc]

According to W3 spec:  http://www.w3.org/TR/html4/interact/forms.html#edef-INPUT (value of attribute "checked" cannot be "true"; must be one of "checked"), the following code is wrong:

<input checked="true" name="sdfs" type="checkbox" value="sdf" />

Instead of checked="true" we should generate checked="checked".

[fredck]

Is the "default IE values" cleanup really needed? It looks more like a bug in the dialog code. It's strange that the defaults are not there when creating a new checkbox, just when editing it.

[garry.yao]

Is the "default IE values" cleanup really needed?

1. It turns out that the cleanup for 'value' is needed, IE's nodeValue for 'value' attr is always 'on', even worse we'll find it on html output as well.
2. It's proved that 'checked' value cleanup is the right place to fix, it's was not long after I read the  nodeName Hell article then it landed in our editor, but with a different scene that in our case is the nodeName of a AttrNode that hurt, instead of a Element - IE always return upper-case for 'CHECKED'.
   

Ticket Test added at :
 http://ckeditor.t/tt/4527/1.html.  http://ckeditor.t/tt/4527/2.html.

[garry.yao]

Fixed with [4547].