Support nil argument as no argument

mrbgems/mruby-enumerator/mrblib/enumerator.rb

@@ -153,9 +153,15 @@ class Enumerator
   #
   def with_index(offset=0)
     return to_enum :with_index, offset unless block_given?
-    raise TypeError, "no implicit conversion of #{offset.class} into Integer" unless offset.respond_to?(:to_int)
+    offset = if offset.nil?
+      0
+    elsif offset.respond_to?(:to_int)
+      offset.to_int
+    else
+      raise TypeError, "no implicit conversion of #{offset.class} into Integer"
+    end
 
-    n = offset.to_int - 1
+    n = offset - 1
     enumerator_block_call do |i|
       n += 1
       yield [i,n]