fix #4862 in _pyio: reset the decoder on seek(0)
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 58f33a2..1c7d1f1 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -1706,6 +1706,8 @@
             f.write(data)
             f.seek(0)
             self.assertEquals(f.read(), data * 2)
+            f.seek(0)
+            self.assertEquals(f.read(), data * 2)
             self.assertEquals(buf.getvalue(), (data * 2).encode(encoding))
 
     def test_read_one_by_one(self):