we only need to check against singleton_class actually
a temporary hack to avoid removing twice. we need to restructure this

we need some way to support ordered mocks, so the data structure
should be redesigned, this is only a temporary workaround.

we need to support this:

obj = Object.new
mock(obj).to_s{ "1" }
mock(obj).to_s{ "2" }
Muack.verify