Uncomments the multiple css class ref-test

This test had been commented out because it was breaking the travis ci
build but was passing locally.

Issue #3681
Update rust-http to pick up latest additions.
auto merge of #3680 : jdm/servo/updatehttp, r=jdm,metajack

Needed for #3596.
Add DOMRefCell<T> for safe borrowing in layout.

This type simply wraps `RefCell<T>` to add the special method,
and introduce the method to return the pointer of the value contained in
itself, for used in layout task.
Use DOMRefCell<T> in Attr.
Use DOMRefCell<T> in Element.
Use DOMRefCell<T> in CharacterData.
auto merge of #3574 : saneyuki/servo/cell, r=jdm

#3050
auto merge of #3682 : gilles-leblanc/servo/issue-3681, r=jdm

This test had been commented out because it was breaking the travis ci
build but was passing locally.

Issue #3681
Add unifying #[dom_struct] attribute (fixes #3667)
Use #[dom_struct] everywhere