fix incorrect handling of return value from ngx_http_read_client_request...

Hi YC:
      By following calling pattern of ngx_http_read_client_request_body() in other core module function such as ngx_http_proxy_handler(). I think it is not necessary to handle NGX_AGAIN return code from ngx_http_read_client_request. Because the inner loop of ngx_http_read_client_request, named ngx_http_do_read_client_request_body() will handle the AGAIN return code for us, verified by my own test case. And  ngx_http_read_client_request_body()  SHOULD NOT be called more than once (http://forum.nginx.org/read.php?29,78035,78349#msg-78349) hence I remove the redundant code that handle NGX_AGAIN from ngx_http_read_client_request_body() .