Avoid unnecessary mutation

I ran into an unexpected behavior when the `getSelectValue` respond body was including a search body:

```
[1] pry(#<NetSuite::Actions::GetSelectValue>)> @response.body
=> {:search_response=>
  {:search_result=>
    {:status=>{:@is_success=>"true"},
     :total_records=>"1",
     :page_size=>"1000",
     :total_pages=>"1",
     :page_index=>"1",
     :search_id=>"WEBSERVICES_3868171_1023201410570198521023020047_dc09b6504f2",
     :record_list=>
#...
```

I realized that `Search` is mutating the auth headers when it uses `update`.