oauth2: Deep copy context client in NewClient

OAuth2 client creation currently doesn't faithfully reuse the client
passed into the context. This causes config settings such as timeout to
be set to Default and may end up to be a gotcha for anyone who sends in
a context client with timeout set assuming that the timeout will be
copied to the new client.

Fix: #368
Change-Id: I4f5f052361ebe07f50fbd694379892833cd1056c
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/180920
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

oauth2.go

@@ -356,11 +356,15 @@ func NewClient(ctx context.Context, src TokenSource) *http.Client {
 	if src == nil {
 		return internal.ContextClient(ctx)
 	}
+	cc := internal.ContextClient(ctx)
 	return &http.Client{
 		Transport: &Transport{
-			Base:   internal.ContextClient(ctx).Transport,
+			Base:   cc.Transport,
 			Source: ReuseTokenSource(nil, src),
 		},
+		CheckRedirect: cc.CheckRedirect,
+		Jar:           cc.Jar,
+		Timeout:       cc.Timeout,
 	}
 }