oauth2: fix warning from go vet

go vet reports the following warning:
oauth2.go:298: golang.org/x/oauth2/internal.ErrorTransport \
composite literal uses unkeyed fields

Change-Id: I576234fcaedb745702fdf8c3431e55128788079d
Reviewed-on: https://go-review.googlesource.com/24991
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

oauth2.go

@@ -295,7 +295,7 @@ func NewClient(ctx context.Context, src TokenSource) *http.Client {
 	if src == nil {
 		c, err := internal.ContextClient(ctx)
 		if err != nil {
-			return &http.Client{Transport: internal.ErrorTransport{err}}
+			return &http.Client{Transport: internal.ErrorTransport{Err: err}}
 		}
 		return c
 	}