Pass the correct argument to Element::create when cloning.

The current code calls as_slice() on the Option, yielding &[DOMString], and
then calls to_string, yielding "[prefix]".
auto merge of #3898 : Ms2ger/servo/prefix-clone, r=jdm

The current code calls as_slice() on the Option, yielding &[DOMString], and
then calls to_string, yielding "[prefix]".