LP#1367926: various improvements and bugfixes

LP#1367926: various improvements and bugfixes

- use entityize() so that non-ASCII characters don't break
  XML well-formedness
- when a U2 identifier is given, the list of supported formats
  returned is now accurate
- biblio_record_entry_feed now works; for example,

/opac/extras/unapi?id=tag::U2@biblio_record_entry_feed/1608,454{holdings_xml}&format=mods32

returns a modsCollection of bibs 1608 and 454.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Kathy Lussier <klussier@masslnc.org>