refactored RLMUpdateQueryWithPredicate to only handle NSPredicates
re-add nil predicate check
update RLMPrecondition message
Merge pull request #869 from realm/jp-id-predicate

refactored RLMUpdateQueryWithPredicate to only handle NSPredicates
fix for posible race conditions in tests
Merge pull request #872 from realm/al-race

Fix for posible race conditions in tests
fix for last race condition
Merge pull request #873 from realm/al-race

Fix for last race condition
Merge branch 'master' into al-primary
fix for hash and isEqual
Merge branch 'al-primary' of https://github.com/realm/realm-objc into al-primary