chore: be consistent and use lower case headers everywhere (#86)

Author: stainless-app[bot]

lib/orb/client.rb

@@ -67,7 +67,7 @@ class Client < Orb::BaseClient
     private def auth_headers
       return {} if @api_key.nil?
 
-      {"Authorization" => "Bearer #{@api_key}"}
+      {"authorization" => "Bearer #{@api_key}"}
     end
 
     # Creates and returns a new client for interacting with the API.