merging / reimplementing r68532 from the trunk to Py3k

merging / reimplementing r68532 from the trunk to Py3k

Enable buffering for HTTPResponse's fp.  read() behaves identically for buffered and non-buffered IO.  read(n) also won't block if n bytes are availble on the socket.  There is therefore no reason not to use buffering.  The reason 2.x disables buffering by default, that some clients may be accessing the underlying socket directly and so bypass the buffering buffer, doesn't apply in 3.x with its redesigned IO library.
See issue 4448 and issue 4879