fix: don't use indifferent access for job params

All jobs refer to parameters using string keys.

closes #2477
closes #2714
closes #2476
closes #2500

lib/postal/job.rb

@@ -5,7 +5,7 @@ class Job
 
     def initialize(id, params = {})
       @id = id
-      @params = params.with_indifferent_access
+      @params = params
       on_initialize
     end