Merge pull request #37 from stacktic/DropboxAuthFix

Add Dropbox to the providers not supporting Auth headers.
diff --git a/oauth2.go b/oauth2.go
index a60b53a..3ac7c7e 100644
--- a/oauth2.go
+++ b/oauth2.go
@@ -285,7 +285,8 @@
 	if strings.HasPrefix(tokenURL, "https://accounts.google.com/") ||
 		strings.HasPrefix(tokenURL, "https://github.com/") ||
 		strings.HasPrefix(tokenURL, "https://api.instagram.com/") ||
-		strings.HasPrefix(tokenURL, "https://www.douban.com/") {
+		strings.HasPrefix(tokenURL, "https://www.douban.com/") ||
+		strings.HasPrefix(tokenURL, "https://api.dropbox.com/") {
 		// Some sites fail to implement the OAuth2 spec fully.
 		return false
 	}