SPDY: avoid creating flush frames.

SPDY: avoid creating flush frames.

Previously, an empty frame object was created for an output chain that contains
only sync or flush empty buffers.  But since 39d7eef2e332 every DATA frame has
the flush flag set on its last buffer, so there's no need any more in additional
flush buffers in the output queue and they can be skipped.

Note that such flush frames caused an incorrect $body_bytes_sent value.