Event pipe: fixed writing cache header to a temp file.

Event pipe: fixed writing cache header to a temp file.

With previous code the p->temp_file->offset wasn't adjusted if a temp
file was written by the code in ngx_event_pipe_write_to_downstream()
after an EOF, resulting in cache not being used with empty scgi and uwsgi
responses with Content-Length set to 0.

Fix it to call ngx_event_pipe_write_chain_to_temp_file() there instead
of calling ngx_write_chain_to_temp_file() directly.