fix(Tree): allows insert paths with shared keys unordered

When adding new nodes into the tree, the lookup mechanism failed
to detect same named parameter was already added to it, raising
an exception.

Example:

    tree = Radix::Tree(Symbol).new
    tree.add "/members/:id/edit", :member_edit
    tree.add "/members/export",   :members_export
    tree.add "/members/:id/videos", :member_videos # Exception

This fix ensures that comparison is done key by key, even if they
are inserted into the tree in a different order.

Closes #9

spec/radix/tree_spec.cr

@@ -233,6 +233,30 @@ module Radix
           tree.root.children[0].children[0].key.should eq("/:subcategory")
         end
 
+        it "does allow same named parameter in different order of insertion" do
+          tree = Tree(Symbol).new
+          tree.add "/members/:id/edit", :member_edit
+          tree.add "/members/export", :members_export
+          tree.add "/members/:id/videos", :member_videos
+
+          # /members/
+          #         +-export      (:members_export)
+          #         \-:id/
+          #              +-videos (:members_videos)
+          #              \-edit   (:members_edit)
+          tree.root.key.should eq("/members/")
+          tree.root.children.size.should eq(2)
+
+          # first level children nodes
+          tree.root.children[0].key.should eq("export")
+          tree.root.children[1].key.should eq(":id/")
+
+          # inner children
+          nodes = tree.root.children[1].children
+          nodes[0].key.should eq("videos")
+          nodes[1].key.should eq("edit")
+        end
+
         it "does not allow different named parameters sharing same level" do
           tree = Tree(Symbol).new
           tree.add "/", :root