Fixes skipping object callback filters

This allows you to skip callbacks that are defined by objects, e.g. for `ActionController`:

    skip_after_filter MySpecialFilter

Previously this didn't work due to a bug in how Rails compared callbacks in `Callback#matches?`. When a callback is compiled, if it's an object filter (i.e. not a method, proc, etc.), `Callback` now defines a method on `@klass` that is derived from the class name rather than `@callback_id`. So, when `skip_callback` tries to find the appropriate callback to remove, `Callback` can regenerate the method name for the filter object and return the correct value for `Callback#matches?`.