I am using selenium-htmlunit-driver-2.4.0 and I want to get all selected options in
a select.

The test fail and looking at the code it seems to be wrong:

In HtmlUnitWebElement:

public boolean isSelected() {
    assertElementNotStale();

    if (element instanceof HtmlInput) {
      return ((HtmlInput) element).isChecked();
    } else if (element instanceof HtmlOption) {
      return ((HtmlOption) element).hasAttribute("selected");
    }

[...]
}

return ((HtmlOption) element).hasAttribute("selected");

should be replaced by

return ((HtmlOption) element).isSelected();

Indeed when you have a select with no options having selected attribute, the first
option is selected by default.

Example:
        <form id="form6">
            <select name="select1">
            <option value="1">one</option>
            <option value="2">two</option>
            <option value="3">three</option>
            </select>
        </form>

Select.getAllSelectedOptions() should return option "one".