Use 'match' instead of 'if'.
Enable to update id attribute for Document.getElementById(). (#740)
Use 'DOMString' to define the type insteand of '~str'.

At now, 'DOMString' is equal to '~str'.
https://github.com/mozilla/servo/blob/155befe10dc56cfb2dfbf0cca7b652293dba9753/src/components/script/dom/bindings/utils.rs#L119
auto merge of #1302 : saneyuki/servo/getid, r=jdm

#740