bring mg_strdup() back

src/rpc.c

@@ -5,8 +5,7 @@ void mg_rpc_add(struct mg_rpc **head, struct mg_str method,
                 void (*fn)(struct mg_rpc_req *), void *fn_data) {
   struct mg_rpc *rpc = (struct mg_rpc *) calloc(1, sizeof(*rpc));
   if (rpc != NULL) {
-    rpc->method.buf = mg_mprintf("%.*s", method.len, method.buf);
-    rpc->method.len = method.len;
+    rpc->method = mg_strdup(method);
     rpc->fn = fn;
     rpc->fn_data = fn_data;
     rpc->next = *head, *head = rpc;