[open3] only close streams if a block is passed

Ref: 5429deb075beb9a2b67adae269bbac16325876d1

The previous change totally borke `popen2e` in this usage:

```ruby
require 'open3'
stdin, stdout, process = Open3.popen2e("cat")
puts stdout.read
```

1 files changed, 6 insertions, 4 deletions

diff --git a/lib/open3.rb b/lib/open3.rb
index ada451c249..3ee81c30ac 100644
--- a/lib/open3.rb
+++ b/lib/open3.rb
@@ -207,10 +207,12 @@ module Open3
 
     popen_run(cmd, opts, [in_r, out_w], [in_w, out_r], &block)
   ensure
-    in_r.close
-    in_w.close
-    out_r.close
-    out_w.close
+    if block
+      in_r.close
+      in_w.close
+      out_r.close
+      out_w.close
+    end
   end
   module_function :popen2e