Fix infinite recursion (solution by @nicomahler)

Closes igrigorik#19

lib/decisiontree/id3_tree.rb

@@ -58,6 +58,7 @@ def id3_train(data, attributes, default, _used={})
       # 2. Pick best attribute
       # 3. If attributes all score the same, then pick a random one to avoid infinite recursion.
       performance = attributes.collect { |attribute| fitness_for(attribute).call(data, attributes, attribute) }
+      return data.first.last if performance.all? { |a, b| a.to_f <= 0 }
       max = performance.max { |a,b| a[0] <=> b[0] }
       min = performance.min { |a,b| a[0] <=> b[0] }
       max = performance.sample if max[0] == min[0]