bnc#885548: Set the date time mode to non-fixed after the import is done.

So that the new revisions will correctly record time stamps.

Change-Id: I7d9b26bed04a95ce2652224faa0f214562818970

sc/source/filter/oox/revisionfragment.cxx

@@ -270,6 +270,7 @@ void RevisionHeadersFragment::finalizeImport()
     }
 
     pCT->SetUser(aSelfUser); // set the default user to the document owner.
+    pCT->SetUseFixDateTime(false);
     rDoc.SetChangeTrack(pCT.release());
 
     // Turn on visibility of tracked changes.