RubySpec for creating a Proc using the block from an enclosing method

This patch adds two matching RubySpec examples for `Proc.new` and `Kernel#proc` that specify their behavior when called without an explicit block. See the context at #3202.

`Kernel#proc` is effectively an alias for `Proc.new`, so it might make sense to move the examples in `spec/ruby/core/proc/new_spec.rb` into a set of shared examples that are applied for both methods, but I opted not to do that without discussing it first and to keep this patch simple and small.