FIX: FIX: Correctly parse Mandrill webhooks (#17116)

Mandrill sends webhook messages as a URL-encoded body, with the
mandrill_events property as an encoded JSON array.

app/controllers/webhooks_controller.rb

@@ -49,7 +49,7 @@ class WebhooksController < ActionController::Base
   end
 
   def mandrill
-    events = params["mandrill_events"]
+    events = JSON.parse(params["mandrill_events"])
     events.each do |event|
       message_id = event.dig("msg", "metadata", "message_id")
       to_address = event.dig("msg", "email")

spec/requests/webhooks_controller_spec.rb

@@ -142,7 +142,7 @@ RSpec.describe WebhooksController do
               "message_id" => message_id
             }
           }
-        }]
+        }].to_json
       }
 
       expect(response.status).to eq(200)