gh-117151: optimize BufferedWriter(), do not buffer writes that are the buffer size (GH-118037)

BufferedWriter() was buffering calls that are the exact same size as the buffer. it's a very common case to read/write in blocks of the exact buffer size.

it's pointless to copy a full buffer, it's costing extra memory copy and the full buffer will have to be written in the next call anyway.

Co-authored-by: rmorotti <romain.morotti@man.com>