Fix and adjust some specs for Hash#reject!

* Add returning Enumerator specs for frozen instance via `it _behaves_like`
* Adjust conflict block `before`
* Fix a spec of "is equivalent to delete_if if changes are made"
  That is comparing same object after side effect method.
* Remove dupulicated specs
  ( "is equivalent to delete_if if changes are made" including
    "processes entries with the same order as delete_if" specs )
Fix Hash#reject! on frozen objects
Merge pull request #2311 from kachick/fix-hash-reject

Hash#reject! on a frozen object should return an Enumerator when no block given