Fixes bug in IE, where all attributes have empty value after sanitizing.

In `_sanitizeAttributes`, the attribute is referenced in a variable
(`tmp`) and removed from `currentNode`.
In IE, after the attribute is removed from `currentNode`, the value in
`tmp` variable is also removed. So instead of just getting the attribute
from `currentNode`, clone `currentNode` and get the attribute from the
`clonedNode`.