Longest common prefix behaviour seems invalid

[liona24]

Hey,

currently the longest_common_prefix function behaves like this:

let mut trie = qp_trie::Trie::new();
let a = vec![1];
let b = vec![1, 2, 3];
trie.insert(a.as_slice(), 1111);
trie.insert(b.as_slice(), 2222);
assert_eq!(
    trie.longest_common_prefix([1u8, 2u8].as_slice()),
    &[1u8, 2u8]
);

But I would I expect it to return [1u8] since this is the only valid prefix with key in the map ([1, 2] is not a valid prefix as it has no value in the map).