Use default client rather than a new client instance.

google/google.go

@@ -76,7 +76,7 @@ func (c *ComputeEngineConfig) FetchToken(existing *oauth2.Token) (token *oauth2.
 		return
 	}
 	req.Header.Add("X-Google-Metadata-Request", "True")
-	resp, err := (&http.Client{}).Do(req)
+	resp, err := http.DefaultClient.Do(req)
 	if err != nil {
 		return
 	}

jwt.go

@@ -100,8 +100,7 @@ func (c *JWTConfig) FetchToken(existing *Token) (token *Token, err error) {
 	v.Set("assertion", payload)
 
 	//  Make a request with assertion to get a new token.
-	client := http.Client{}
+	resp, err := http.DefaultClient.PostForm(c.aud, v)
-	resp, err := client.PostForm(c.aud, v)
 	if err != nil {
 		return nil, err
 	}

oauth2.go

@@ -206,7 +206,7 @@ func (c *Config) exchange(exchangeCode string) (*Token, error) {
 func (c *Config) updateToken(tok *Token, v url.Values) error {
 	v.Set("client_id", c.opts.ClientID)
 	v.Set("client_secret", c.opts.ClientSecret)
-	r, err := (&http.Client{}).PostForm(c.tokenURL, v)
+	r, err := http.DefaultClient.PostForm(c.tokenURL, v)
 	if err != nil {
 		return err
 	}