MapScan replaces underlying string pointer with a string

[dkropachev]

func TestMapScanWithRefMap2(t *testing.T) {
	session := createSession(t)
	defer session.Close()
	if err := createTable(session, `CREATE TABLE gocql_test.scan_map_ref_table (
			testtext       text PRIMARY KEY,
			testint        int,
		)`); err != nil {
		t.Fatal("create table:", err)
	}
	m := make(map[string]interface{})
	m["testtext"] = "testtext"
	m["testint"] = 100

	if err := session.Query(`INSERT INTO scan_map_ref_table (testtext, testint) values (?,?)`,
		m["testtext"], m["testint"]).Exec(); err != nil {
		t.Fatal("insert:", err)
	}

	var testText string
	ret := map[string]interface{}{
		"testtext": &testText,
		// testint is not set here.
	}
	iter := session.Query(`SELECT * FROM scan_map_ref_table`).Iter()
	if ok := iter.MapScan(ret); !ok {
		t.Fatal("select:", iter.Close())
	}

	if ret["testtext"].(*string) != &testText {
		t.Fatal("returned testtext is not the same string")
	}

	iter = session.Query(`SELECT * FROM scan_map_ref_table`).Iter()
	if ok := iter.MapScan(ret); !ok {
		t.Fatal("select:", iter.Close())
	}
}

Ends up with:

=== RUN   TestMapScanWithRefMap2
--- FAIL: TestMapScanWithRefMap2 (0.08s)
panic: interface conversion: interface {} is string, not *string [recovered]
	panic: interface conversion: interface {} is string, not *string