[Foreign_link] Change the type of "credentials" to blob

TwitterOAuthClient::packToken uses \0 as a delimeter which can cause issues on
TEXT or VARCHAR.

plugins/TwitterBridge/actions/twitterauthorization.php

@@ -228,7 +228,7 @@ class TwitterauthorizationAction extends FormAction
 
         $creds = TwitterOAuthClient::packToken($access_token);
 
-        $flink->credentials = $creds;
+        $flink->credentials = DB_DataObject_Cast::blob($creds);
         $flink->created     = common_sql_now();
 
         // Defaults: noticesync on, everything else off