Fix Page::getCollectionPathFromID

getCollectionPathFromID should return the canonical path of a page.

This fix will fix this, by ordering by ppIsCanonical in descendant order,
so that the first row (ie the returned one) is the one with
ppIsCanonical = 1
Merge pull request #1567 from mlocati/fix-page-getcollectionpathfromid

Fix Page::getCollectionPathFromID