Add specs for Array#keep_if, Array#select! on frozen objects
Add spec for Array#delete_if on frozen objects
Add spec for Array#reject! on frozen objects
Add a spec for String#split retaining the original string's encoding
Add specs for Enumerable#chunk when block returns a reserved value
Add specs for Enumerator#peek_values
Write specs for Enumerator#next_values
Add spec for File.expand_path not returning frozen strings

Extract from #2288
Add specs for String#upto on sequence of numbers
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 )
Add a spec for Symbol#to_proc