`Trie::remove()` sometimes sometime removes more than one element

[dwrensha]

extern crate radix_trie;
use radix_trie::Trie;

fn main() {
    let mut trie = Trie::new();
    trie.insert("a", ());
    trie.insert("p", ());
    trie.remove(&"a");
    assert_eq!(trie.len(), 1); 
}

thread '<main>' panicked at 'assertion failed: `(left == right)` (left: `0`, right: `1`)',

I think this occurs whenever the root node goes from having 2 children to having only 1 child. Trie::delete_node() tries to replace the root node with its only child, but Trie::remove() ignores the DeleteAction that would make this work.

[dwrensha]

Yay, thanks for fixing!