Add application/atom+xml as an XML document type.

Change-Id: I9fcf438f3cafcb3ab3a5b6dc0f72ed2561e5a94f
Reviewed-on: https://code.wireshark.org/review/3049
Reviewed-by: Guy Harris <guy@alum.mit.edu>
(cherry picked from commit 52a4f3d121765ce758c16137b03e7b9d2e15908c)
Reviewed-on: https://code.wireshark.org/review/3050