Spy class methods correctly

[igrep]

I forgot to reset received_message when spy_into an already-spied object. So seems crispy can't log class methods correctly.

e.g.

class Hoge
  def self.foo
  end
end

Crispy.spy_into Hoge
Hoge.foo

Crispy.spy_into Hoge
Hoge.foo

spy(Hoge).received_messages
#=> the doubled received messages!