Get most Enumerator::Product specs passing

Remaining specs are for #inspect which is not yet implemented.

See marcandre/backports#194

core/src/main/ruby/jruby/kernel/enumerator.rb

@@ -487,11 +487,14 @@ def Enumerator.product(*enums, **kwargs, &block)
 class Enumerator::Product < Enumerator
   def initialize(*enums, **nil)
     @__enums = enums
+    self
   end
 
   def each(&block)
-    return self unless block
-    __execute(block, [], @__enums)
+    if block
+      __execute(block, [], @__enums)
+    end
+    self
   end
 
   def __execute(block, values, enums)
@@ -525,4 +528,20 @@ def rewind
     end
     self
   end
+
+  private def initialize_copy(other)
+    return self if self.equal?(other)
+
+    raise TypeError if !(Product === other)
+
+    super(other)
+
+    other_enums = other.instance_variable_get(:@__enums)
+
+    raise ArgumentError.new("uninitialized product") unless other_enums
+
+    @__enums = other_enums
+
+    self
+  end
 end