#4833 isXml function throws an error in IE when used in combination with an iframe

#4833 isXml function throws an error in IE when used in combination with an iframe


Since posting links to tickets with patches here pays off I'm doing it
aswell!
http://dev.jquery.com/ticket/4833
http://dev.jquery.com/attachment/ticket/4833/isXml.patch
It just needs a review... altought its just 1 lil extra test in an if
statement and a commit.