XWIKI-11450: Title of Blog.BlogSheet does not get updated when updating from 6.1 to 6.x, x>1

* actually modifications in properties merged as objects have
  not been signaled to the mergeResult, causing them not to be persisted
  if they are the only changes to that document

Conflicts:

	xwiki-platform-core/xwiki-platform-oldcore/src/test/java/com/xpn/xwiki/internal/merge/MergeUtilsTest.java
  (with XWIKI-11454, easily resolved)
XWIKI-11450: Title of Blog.BlogSheet does not get updated when updating from 6.1 to 6.x, x>1

* avoid triggering a merge in import just because
  the imported page has no syntax and thus falls back to xwiki/1.0
  while the default in the wiki is plain/1.0