Fix incorrect return type

`Result#key` is expected to be `String`, yet Crystal considered
that by branching is possible result to be `nil`.

This was caused by *return-early* approach:

    def foo
      return @foo if @foo

      @foo = "awesome"
    end

    pp typeof(foo) # => String?

Changed to instead use memoization technique.

src/radix/result.cr

@@ -61,15 +61,13 @@ module Radix
     # # => ""
     # ```
     def key : String
-      return @key if @key
-
-      key = String.build { |io|
+      @key ||= begin
+        String.build { |io|
           @nodes.each do |node|
             io << node.key
           end
         }
-
-      @key = key
+      end
     end
 
     # Adjust result information by using the details of the given `Node`.