oauth2/internal: fix the broken ini parsing test

Change-Id: I1032b759e036f57bc49eec951d08d8b22728c1a0
Reviewed-on: https://go-review.googlesource.com/3117
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

internal/oauth2_test.go

@@ -45,7 +45,7 @@ empty=
 ;comment=true
 `,
 			map[string]map[string]string{
-				"a": map[string]string{},
+				"": map[string]string{},
 			},
 		},
 	}