nginx: Disable on-disk buffering for proxy responses

From the docs [1]:

  When buffering of responses from the proxied server is enabled, and
  the whole response does not fit into the buffers set by the
  proxy_buffer_size and proxy_buffers directives, a part of the
  response can be saved to a temporary file. This directive sets the
  maximum size of the temporary file. The size of data written to the
  temporary file at a time is set by the proxy_temp_file_write_size
  directive.

  The zero value disables buffering of responses to temporary files.

Note that this doesn't disable buffering of client uploads.  There's
no way to do that in Nginx at the moment [2] (without external patches
[3]).

[1]: http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_max_temp_file_size
[2]: http://trac.nginx.org/nginx/ticket/251
[3]: http://forum.nginx.org/read.php?2,244680,253746#msg-253746