Fix destructive methods for selecting options

Both `select_option` and `unselect_option` were modifying the
`selected` attribute of their target `<option>` element. This attribute
is meant to be used solely as the marker for which `<option>`(s) is
(are) the default selection(s) in a (multi-)`<select>` element. The
actual _selectedness_ of an `<option>` should be tracked using its
`selected` **property**. Read [the spec][] for more info.

This change removes any code which modified the `selected` attribute of
`<option>` elements, which leaves only the code that modifies the
`selected` property. In addition, `Node#value` needed to be changed to
return `<option>` elements whose selectedness is true rather than just
those with a `selected` attribute.

The tests introduced in the previous commit now pass.

[the spec]: http://dev.w3.org/html5/spec/Overview.html#the-option-element

lib/capybara/driver/webkit/node.rb

@@ -22,11 +22,7 @@ def [](name)
 
     def value
       if multiple_select?
-        self.find(".//option").select do |option|
-          option["selected"] == "selected"
-        end.map do |option|
-          option.value
-        end
+        self.find(".//option").select(&:selected?).map(&:value)
       else
         invoke "value"
       end

src/capybara.js

@@ -196,13 +196,11 @@ Capybara = {
 
   selectOption: function(index) {
     this.nodes[index].selected = true;
-    this.nodes[index].setAttribute("selected", "selected");
     this.trigger(index, "change");
   },
 
   unselectOption: function(index) {
     this.nodes[index].selected = false;
-    this.nodes[index].removeAttribute("selected");
     this.trigger(index, "change");
   },