Fix arrange method dropping nodes without parents

Addresses #362

lib/ancestry/class_methods.rb

@@ -40,19 +40,17 @@ def arrange options = {}
     # {node => children}, where children = {} if the node has no children
     def arrange_nodes(nodes)
       arranged = ActiveSupport::OrderedHash.new
-      min_depth = Float::INFINITY
+      nodes_ids = nodes.map(&:id)
+      min_depth = nodes.map(&:depth).min
       index = Hash.new { |h, k| h[k] = ActiveSupport::OrderedHash.new }
 
       nodes.each do |node|
         children = index[node.id]
         index[node.parent_id][node] = children
 
-        depth = node.depth
-        if depth < min_depth
-          min_depth = depth
-          arranged.clear
+        if node.depth == min_depth || !nodes_ids.include?(node.parent_id)
+          arranged[node] = children
         end
-        arranged[node] = children if depth == min_depth
       end
 
       arranged