Replace many uses of Attr::Value() by Attr::value()

The later only borrows the attribute, without copying its value as a string.

components/script/dom/htmlformelement.rs

@@ -314,7 +314,7 @@ impl HTMLFormElement {
                         _ => Some(FormDatum {
                             ty: ty,
                             name: name,
-                            value: input.Value()
+                            value: value
                         })
                     }
                 }