make StringIO like other file objects in that readline(-1) has no effect #7348
diff --git a/Lib/test/test_StringIO.py b/Lib/test/test_StringIO.py
index f5a177e..d10255d 100644
--- a/Lib/test/test_StringIO.py
+++ b/Lib/test/test_StringIO.py
@@ -28,6 +28,8 @@
         eq(self._fp.read(10), self._line[:10])
         eq(self._fp.readline(), self._line[10:] + '\n')
         eq(len(self._fp.readlines(60)), 2)
+        self._fp.seek(0)
+        eq(self._fp.readline(-1), self._line + '\n')
 
     def test_writes(self):
         f = self.MODULE.StringIO()