Allow OAuth endpoints on www.tumblr.com using RequestHandler

RequestHandler: Do not insert "http://api.tumblr.com/" to the given path if the path is already a full url. This allows us to use it for OAuth endpoints (e.g. oauth/request_token). These endpoints are on "www.tumblr.com" and not on the "api" subdomain. Example usage:

    $req = $client->requestHandler->request('POST', 'http://www.tumblr.com/oauth/request_token', [
      'oauth_callback' => '...',
    ]);