return "<nil>" when calling String() on a nil bytes.Buffer.

R=rsc
CC=go-dev
http://go/go-review/1016005
diff --git a/src/pkg/bytes/buffer_test.go b/src/pkg/bytes/buffer_test.go
index f82c984..c364eec 100644
--- a/src/pkg/bytes/buffer_test.go
+++ b/src/pkg/bytes/buffer_test.go
@@ -232,3 +232,11 @@
 	}
 	empty(t, "TestMixedReadsAndWrites (2)", &buf, s, make([]byte, buf.Len()));
 }
+
+
+func TestNil(t *testing.T) {
+	var b *Buffer;
+	if b.String() != "<nil>" {
+		t.Error("expcted <nil>; got %q", b.String());
+	}
+}