How to pass storeView parameter to request (MagentoAPI, SOAP) using 'savon' gem?

Question

Ruby on Rails

the response to such a request is:
$response = $client.call(:call){message :session => $session, :method=> 'category.tree', "parent_id"=> 1 }.body[:call_response][ :call_return][:item]
but I need a category tree for a specific storeView!
$response = $client.call(:call){message :session => $session, :method=> 'category.tree', "parent_id"=> 1, "storeView"=> 10 }.body[:call_response] [:call_return][:item]
This request comes up with this:
Savon::SOAPFault: (SOAP-ENV:Client) Error cannot find parameter
how to pass storeView's id parameter?