Fix userdata re-use after Remove

See also: https://github.com/valyala/fasthttp/pull/1298

userdata.go

@@ -81,7 +81,7 @@ func (d *userData) Remove(key string) {
 		kv := &args[i]
 		if string(kv.key) == key {
 			n--
-			args[i] = args[n]
+			args[i], args[n] = args[n], args[i]
 			args[n].value = nil
 			args = args[:n]
 			*d = args

userdata_test.go

@@ -104,3 +104,18 @@ func TestUserDataDelete(t *testing.T) {
 	}
 
 }
+
+func TestUserDataSetAndRemove(t *testing.T) {
+	var (
+		u        userData
+		shortKey = "[]"
+		longKey  = "[  ]"
+	)
+
+	u.Set(shortKey, "")
+	u.Set(longKey, "")
+	u.Remove(shortKey)
+	u.Set(shortKey, "")
+	testUserDataGet(t, &u, []byte(shortKey), "")
+	testUserDataGet(t, &u, []byte(longKey), "")
+}