raise ArgumentError in Enumerator#new in no given blocks
Enumerator#new allows this code (no block given):
obj = Object.new Enumerator.new(obj)
with a warning (since ruby-2.0.0-preview2):
-e:1: warning: Enumerator.new without a block is deprecated; use Object#to_enum instead
I think it's okay to remove this behaviour.
And I think that it seems natural to raise ArgumentError if no block is given.