Redefine #respond_to_missing with #method_missing

Initial attempt:

class User
  def method_missing(name, *args, &block)
    if "#{name}" =~ /foo/
      'exists'
    else
      super
    end
  end
end

> u = User.new
=> #<User:0x007fdf3b152c60>
> u.foo
=> exists
> u.method(:foo)
=> NameError: undefined method `foo' for class `User'

User.class_eval do
  def respond_to_missing?(name, include_private = false)
    "#{name}" =~ /foo/ || super
  end
end

> u = User.new
=> #<User:0x007ffcda847f90>
> u.method(:foo)
=> #<Method: User#foo>