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 fixed)