add test for previous change to io.c.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@5149 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_file.rb

@@ -40,4 +40,5 @@ class TestFile < Test::Unit::TestCase
     f.rewind
     yield f
   end
+  alias open_file_rw open_file
 end

test/ruby/ut_eof.rb

@@ -16,6 +16,16 @@ module TestEOF
     }
   end
 
+  def test_eof_0_rw
+    return unless respond_to? :open_file_rw
+    open_file_rw("") {|f|
+      assert_equal("", f.read)
+      assert_equal(nil, f.read)
+      assert_equal(0, f.syswrite(""))
+      assert_equal(nil, f.read)
+    }
+  end
+
   def test_eof_1
     open_file("a") {|f|
       assert_equal("", f.read(0))