oauth2: replace a magic number with AuthStyleUnknown

Change-Id: I7e08428b87f141fa6d2932b7d60b3e726a454986
GitHub-Last-Rev: 0aef667f234844d4784c92b9100054041b343840
GitHub-Pull-Request: golang/oauth2#646
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/496075
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

internal/token.go

diff --git a/internal/token.go b/internal/token.go
index f1bd44b..b417456 100644
--- a/internal/token.go
+++ b/internal/token.go
@@ -216,7 +216,7 @@
 }
 
 func RetrieveToken(ctx context.Context, clientID, clientSecret, tokenURL string, v url.Values, authStyle AuthStyle, styleCache *AuthStyleCache) (*Token, error) {
-	needsAuthStyleProbe := authStyle == 0
+	needsAuthStyleProbe := authStyle == AuthStyleUnknown
 	if needsAuthStyleProbe {
 		if style, ok := styleCache.lookupAuthStyle(tokenURL); ok {
 			authStyle = style