`Enumerator::Chain#rewind` shouldn't rewind elements aren't iterated

### Example:

  ```ruby
  # example.rb
  e = [1]
  def e.rewind; p :r end
  c = e.chain(e)
  c.each{break c}.rewind
  ```

#### Before this patch:

  ```terminal
  $ bin/mruby example.rb
  :r
  :r
  ```

#### After this patch (same as Ruby):

  ```terminal
  $ bin/mruby example.rb
  :r
  ```

1 files changed, 16 insertions, 5 deletions

diff --git a/mrbgems/mruby-enum-chain/test/enum_chain.rb b/mrbgems/mruby-enum-chain/test/enum_chain.rb
index 1d3d691ca..45bbc9a77 100644
--- a/mrbgems/mruby-enum-chain/test/enum_chain.rb
+++ b/mrbgems/mruby-enum-chain/test/enum_chain.rb
@@ -76,13 +76,24 @@ assert("Enumerator::Chain#size") do
 end
 
 assert("Enumerator::Chain#rewind") do
-  rewound = []
+  rewound = nil
   e1 = [1, 2]
   e2 = (4..6)
-  (class << e1; self end).define_method(:rewind) { rewound << __id__ }
-  (class << e2; self end).define_method(:rewind) { rewound << __id__ }
-  c = e1.chain(e2).each{}.rewind
-  assert_equal [e2.__id__, e1.__id__], rewound
+  (class << e1; self end).define_method(:rewind) { rewound << self }
+  (class << e2; self end).define_method(:rewind) { rewound << self }
+  c = e1.chain(e2)
+
+  rewound = []
+  c.rewind
+  assert_equal [], rewound
+
+  rewound = []
+  c.each{break c}.rewind
+  assert_equal [e1], rewound
+
+  rewound = []
+  c.each{}.rewind
+  assert_equal [e2, e1], rewound
 end
 
 assert("Enumerator::Chain#+") do