Don't call Scheduler#close if it doesn't exist.

Merged: https://github.com/ruby/ruby/pull/3612

scheduler.c

@@ -39,7 +39,9 @@ Init_Scheduler(void)
 
 VALUE rb_scheduler_close(VALUE scheduler)
 {
-    return rb_funcall(scheduler, id_close, 0);
+    if (rb_respond_to(scheduler, id_close)) {
+        return rb_funcall(scheduler, id_close, 0);
+    }
 }
 
 VALUE

test/fiber/test_scheduler.rb

@@ -49,4 +49,12 @@ class TestFiberScheduler < Test::Unit::TestCase
     end
     RUBY
   end
+
+  def test_optional_close
+    thread = Thread.new do
+      Thread.current.scheduler = Object.new
+    end
+    
+    thread.join
+  end
 end