SPDY: fixed possible premature close of stream.

The "delayed" flag always should be set if there are unsent frames,
but this might not be the case if ngx_http_spdy_body_filter() was
called with NULL chain.

As a result, the "send_timeout" timer could be set on a stream in
ngx_http_writer().  And if the timeout occurred before all the stream
data has been sent, then the request was finalized with the "client
timed out" error.