fix pkey error message

Author: chronolaw

lib/resty/core/socket.lua

@@ -169,7 +169,7 @@ local function setclientcert(cosocket, cert, pkey)
     end
 
     if type(pkey) ~= "cdata" then
-        return nil, "bad pkey arg: cdata expected, got " .. tostring(pkey)
+        return nil, "bad pkey arg: cdata expected, got " .. type(pkey)
     end
 
     cosocket[SOCKET_CLIENT_CERT_INDEX] = cert