fix: add []= delegator (#590)

This allows code that updates the payload in a before_event block to look a little nicer. :)

Author: stympy

lib/honeybadger/event.rb

@@ -13,7 +13,7 @@ class Event
     # The payload data of the event
     attr_reader :payload
 
-    def_delegator :payload, :[]
+    def_delegators :payload, :[], :[]=
 
     # @api private
     def initialize(event_type_or_payload, payload={})