Avoid buffered source/sink creation when RawSource::buffered/RawSink::buffered invoked on Source/Sink

[fzhinkin]

Currently, RawSource::buffered/RawSink::buffered always create a new Source/Sink even when invoked on Source/Sink instance.
It's convenient for testing, and there are arguments why doing it differently may cause issues (#136 (comment)).

However, it may be worth adding a separate set of extensions (or adding a parameter to RawSink::buffered/RawSource::buffered) allowing to bypass of a new buffered source/sink creation for buffered invoked on Source/Sink.