[open3] only close streams if a block is passed

Ref: 5429deb075

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

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

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
 

test/test_open3.rb

@@ -149,6 +149,17 @@ class TestOpen3 < Test::Unit::TestCase
     }
   end
 
+  def test_popen2e_noblock
+    i, o, t = Open3.popen2e(RUBY, '-e', 'STDOUT.print STDIN.read')
+    i.print "baz"
+    i.close
+    assert_equal("baz", o.read)
+  ensure
+    i.close
+    o.close
+    t.join
+  end
+
   def test_capture3
     o, e, s = Open3.capture3(RUBY, '-e', 'i=STDIN.read; print i+"o"; STDOUT.flush; STDERR.print i+"e"', :stdin_data=>"i")
     assert_equal("io", o)