Avoid unnecessary mutation

[greggroth]

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.

[iloveitaly]

Thanks! Merged with 030d2c8