@Test public void combinerShouldModifyKey() {

    initPrefs("vehicle", "bicycle");
    PrefMigrator.migrate(prefs, COMBINER_MODIFIER);
    assertPrefs("vehicle", "bicycle");
    initPrefs("vehicle", "bicycle", "fast", true);
    PrefMigrator.migrate(prefs, COMBINER_MODIFIER);
    assertPrefs("vehicle", "fast_bicycle");
    initPrefs("vehicle", "car");
    PrefMigrator.migrate(prefs, COMBINER_MODIFIER);
    assertPrefs("vehicle", "car");
    initPrefs("vehicle", "car", "fast", true);
    PrefMigrator.migrate(prefs, COMBINER_MODIFIER);
    assertPrefs("vehicle", "fast_car");
    initPrefs("vehicle", "airplane", "fast", true);
    PrefMigrator.migrate(prefs, COMBINER_MODIFIER);
    assertPrefs("vehicle", "airplane", "fast", true);
}