OPML export: ampersand (and likely angle brackets) are not escaped for title element

How to test:
1) Move to any node (preferably with children) that has an & in the content.
2) Press F5 (command line) and enter `export_opml_branch test.opml`
3) Using xmllint, the exported OPML document is not well-formed.
4) Viewing in a text editor, note the head/title element has unescaped characters.

Inserting `<` in a node, it looks like export drops it.