Fix response body accessors for bodyless and rebuilt responses

[hyperxpro]

Motivation:

ResponseBuilder#build() hands out the list it accumulates into, and AsyncCompletionHandler resets that builder on every onStatusReceived. A handler reused for a second request empties the first response and refills it with the
second body.

NettyResponse#getResponseBodyAsByteBuf() sizes a composite to the part count, and CompositeByteBuf rejects zero components, so a bodyless response throws instead of returning an empty buffer.

The body view added in #2322 names three of the four holders of its array, and its NettyResponse override replaced the inherited description, dropping the read-only warning from the only published implementation.

Modification:

Copy the parts in build(), floor the composite at one component, name getBodyByteBuffer() as the fourth holder, and restore the inherited text with {@inheritDoc}.

Result:

A response keeps its own body when its builder is reused, a bodyless response yields an empty buffer, and the view names every holder it has.