Brotli: send special buffers down the pipe.
Brotli: send special buffers down the pipe.

Previously, special "flush" buffer was discarded if there was nothing
to flush from Brotli filter, which resulted in HTTP headers not being
flushed to the client in case of upstream read timeout.

This should have been included in 2fc6f12.

Change-Id: I2f7dfb06f67cc69740998281af95d90428dafa8e
Signed-off-by: Piotr Sikora <ba6f3a1a6d7e1ecae91b31e93969cc392fbd0fdd@google.com>

google/ngx_brotli,google/ngx_brotli