2.2 fix merge only paralell contexts in default

a possible fix for issue: https://github.com/magicalpanda/MagicalRecord/issues/613
others have issues with context merging their own saves too f.e. RestKit:
https://github.com/RestKit/RestKit/issues/1351

these delete problems do not always seem to occur, so a unit test is hard, I only have one in my own project currently
to some degree I think this is a bug from apple, but that is a possible work around