HTTP/2: improved handling of END_STREAM in a separate DATA frame.

HTTP/2: improved handling of END_STREAM in a separate DATA frame.

The save body filter saves the request body to disk once the buffer is full.
Yet in HTTP/2 this might happen even if there is no need to save anything
to disk, notably when content length is known and the END_STREAM flag is
sent in a separate empty DATA frame.  Workaround is to provide additional
byte in the buffer, so saving the request body won't be triggered.

This fixes unexpected request body disk buffering in HTTP/2 observed after
the previous change when content length is known and the END_STREAM flag
is sent in a separate empty DATA frame.