Verifies that the actual array contains the given values only once.
Examples :
// array is a factory method to create arrays.
// assertions will pass
assertThat(array("winter", "is", "coming")).containsOnlyOnce("winter");
assertThat(array("winter", "is", "coming")).containsOnlyOnce("coming", "winter");
// assertions will fail
assertThat(array("winter", "is", "coming")).containsOnlyOnce("Lannister");
assertThat(array("Arya", "Stark", "daughter", "of", "Ned", "Stark")).containsOnlyOnce("Stark");
assertThat(array("Arya", "Stark", "daughter", "of", "Ned", "Stark")).containsOnlyOnce("Stark", "Lannister", "Arya");