Values for checkboxes in $form_state do not match input

The latest patch does not apply anymore.

Updated the #8 patch, the patch on #10 applies cleanly. please review.

I created a MR with the same changes in the patch in comment #7.

There is something that is not clear for me, in form_type_checkbox_value().

function form_type_checkbox_value($element, $input = FALSE) {
  if ($input === FALSE) {
    return isset($element['#default_value']) ? $element['#default_value'] : 0;
  }
  else {
    return isset($input) ? $element['#return_value'] : 0;
  }
}

isset($input) would always return TRUE, even when form_type_checkbox_value() is called as form_type_checkbox_value($element) or form_type_checkbox_value($element, NULL).
Given that return isset($input) ? $element['#return_value'] : 0; is only executed when $input is not FALSE, why is not that line simply return $element['#return_value'];? Eventually, if $element['#return_value'] could be not set, that line should be return isset($element['#return_value']) ? $element['#return_value'] : 0;.

The description of the return value needs to be changed too.

The data that will appear in $form_state['values'] for this element, or nothing to use the default.

form_type_checkbox_value() always return a value, even if that value is 0. It is not true it can return nothing. That comment cannot be changed to or nothing to use the default either. That would contrast with the comment inside
form_type_checkbox_value(), which contains the following sentence.

Returning NULL from a value callback means to use the default value, which is not what is wanted when an unchecked checkbox is submitted, so we use integer 0 as the value indicating an unchecked checkbox.

Furthermore, this comment needs to be changed too, since the code is changed.

// Checked checkboxes are submitted with a value (possibly '0' or ''):
// http://www.w3.org/TR/html401/interact/forms.html#successful-controls.
// For checked checkboxes, browsers submit the string version of
// #return_value, but we return the original #return_value. For unchecked
// checkboxes, browsers submit nothing at all, but
// _form_builder_handle_input_element() detects this, and calls this
// function with $input=NULL. Returning NULL from a value callback means to
// use the default value, which is not what is wanted when an unchecked
// checkbox is submitted, so we use integer 0 as the value indicating an
// unchecked checkbox. Therefore, modules must not use integer 0 as a
// #return_value, as doing so results in the checkbox always being treated
// as unchecked. The string '0' is allowed for #return_value. The most
// common use-case for setting #return_value to either 0 or '0' is for the
// first option within a 0-indexed array of checkboxes, and for this,
// form_process_checkboxes() uses the string rather than the integer.

we return the original #return_value is not always true, as $input is returned for programmatically submitted forms.

I did not change the other phrase.
Given the context in which it is used (browsers submit the string version of #return_value, but we return the original #return_value), it is not necessary to say that also $input could be returned.