Bug 16111: (QA follow-up) Few additional corrections

Bug 16111: (QA follow-up) Few additional corrections

[1] Refining the regex on the format parameter in opac-search.pl
[2] Adding a colon to dc:identifier. The Dublin Core specs gives me this
    example:
    Identifier="ISBN:0385424728"
[3] Replacing last occurrence of rss2 in the rss template

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit 2bbd811027172af1adb68f9904071951056ecbda)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>