walk_tree_bfs doesn not implement BFS

[qheath]

the current implementation of walk_tree_bfs actually implements DFS, as evidenced by the fact that it is almost identical to walk_tree_dfs

in other words, walk_tree_bfs node {...} is identical to walk_tree_dfs node { |node,level| node.children.each { |child| ... }}, modulo some corner-cases

I think the tests used in test/acts_as_tree_test.rb are not deep enough to show the difference, but if you take

1
-2
--3
---4
-5
--6
---7

then the current implementation generate

1 2 5 3 4 6 7

while I believe true BFS would generate

1 2 5 3 6 4 7

(4 and 6 are switched)

a correct implementation should use a FIFO to keep tracks of the next generation; I will suggest one later, unless someone beats me to it or explains that I'm mistaken