internal: urlencode client id and secret in header

As per https://tools.ietf.org/html/rfc6749#section-2.3.1 client IDs and secrets must be urlencoded in the authorization header. This patch addresses this by wrapping clientID and clientSecret with url.QueryEscape. A dedicated test for unsafe-url client IDs and secrets has been added as well.

Closes #237

Change-Id: I1f277b52caef4932e14147be8fb1712203da51d0
Reviewed-on: https://go-review.googlesource.com/46473
Reviewed-by: JBD <jbd@google.com>

internal/token.go

@@ -188,7 +188,7 @@ func RetrieveToken(ctx context.Context, clientID, clientSecret, tokenURL string,
 	}
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 	if !bustedAuth {
-		req.SetBasicAuth(clientID, clientSecret)
+		req.SetBasicAuth(url.QueryEscape(clientID), url.QueryEscape(clientSecret))
 	}
 	r, err := ctxhttp.Do(ctx, hc, req)
 	if err != nil {