Prepended modules cause spy to think a method has never been hooked

Steps to reproduce:

``` ruby
class Foo
  class << self
    prepend Bar
  end

  def self.foo
  end
end

module Bar
  def foo
  end
end

class Test < MiniTest::Test
  def test_stuff
    Spy.on(Foo, :bar)
  end
end
```

Spy will continuously complain that `Foo.bar` hasn't been hooked, and will continue to do so for every subsequent test run as it continuously tries to unhook the method.
