Integral#step without arg should loop forever as CRuby does.

mrblib/numeric.rb

@@ -100,11 +100,18 @@ module Integral
   # Calls the given block from +self+ to +num+
   # incremented by +step+ (default 1).
   #
-  def step(num, step = 1, &block)
+  def step(num=nil, step=1, &block)
     raise ArgumentError, "step can't be 0" if step == 0
     return to_enum(:step, num, step) unless block_given?
 
     i = if num.kind_of? Float then self.to_f else self end
+    if num == nil
+      while true
+        block.call(i)
+        i+=step
+      end
+      return self
+    end
     if step > 0
       while i <= num
         block.call(i)