Reset @dirty to false when slicing an instance of SafeBuffer

[bcardarella]

This resolves the warning being throw in actionpack/capture_helper_test#test_flush_output_buffer_concats_output_buffer_to_response

After the OutputBuffer was being flushed by calling [0,0] on it the @dirty instance variable was not initialized in the new instance. This was throwing a warning.

The fix here will set the @dirty instance variable to false on the new instance when the slice method is invoked.

[josevalim]

This is weird. Why is slice not calling initialize in the first place? Is there a Ruby hook that it should call? /cc @tenderlove

[bcardarella]

Not certain, that threw me for a loop as well. That would make things much simpler and it would ensure the super call in the SafeBuffer initializer is called.

[bcardarella]

TBH, I'm not a big fan of the solution I submitted but it was the only one that worked.

[tenderlove]

@josevalim this is the danger of subclassing core classes. It's an optimization that basically all core classes have (and why we should prefer composition over inheritance).