`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, 12 insertions, 7 deletions

diff --git a/mrbgems/mruby-enum-chain/mrblib/chain.rb b/mrbgems/mruby-enum-chain/mrblib/chain.rb
index 55474eb92..43d0926c8 100644
--- a/mrbgems/mruby-enum-chain/mrblib/chain.rb
+++ b/mrbgems/mruby-enum-chain/mrblib/chain.rb
@@ -17,13 +17,19 @@ class Enumerator
     include Enumerable
 
     def initialize(*args)
-      @enums = args
+      @enums = args.freeze
+      @pos = -1
     end
 
     def each(&block)
-      return to_enum unless block_given?
+      return to_enum unless block
 
-      @enums.each { |e| e.each(&block) }
+      i = 0
+      while i < @enums.size
+        @pos = i
+        @enums[i].each(&block)
+        i += 1
+      end
 
       self
     end
@@ -36,11 +42,10 @@ class Enumerator
     end
 
     def rewind
-      i = @enums.size - 1
-      while 0 <= i
-        e = @enums[i]
+      while 0 <= @pos && @pos < @enums.size
+        e = @enums[@pos]
         e.rewind if e.respond_to?(:rewind)
-        i -= 1
+        @pos -= 1
       end
 
       self