read 'x-real-ip' header from nginx in websocket lib. (#1665)

* read 'x-real-ip' header from nginx in websocket lib.

* change default real_ip value to nil

lualib/http/websocket.lua

@@ -139,6 +139,9 @@ local function read_handshake(self, upgrade_ops)
         end
     end
 
+    -- read 'x-real-ip' header from nginx
+    self.real_ip = header["x-real-ip"]
+
     -- response handshake
     local accept = crypt.base64encode(crypt.sha1(sw_key .. self.guid))
     local resp = "HTTP/1.1 101 Switching Protocols\r\n"..
@@ -497,6 +500,11 @@ function M.addrinfo(id)
     return ws_obj.addr
 end
 
+function M.real_ip(id)
+    local ws_obj = assert(ws_pool[id])
+    return ws_obj.real_ip
+end
+
 function M.close(id, code ,reason)
     local ws_obj = ws_pool[id]
     if not ws_obj then