Pass a String to ActiveJob (rather than a SafeBuffer)

This provides compatibility with Sidekiq as an ActiveJob adapter (because [Sidekiq only allows native JSON types][1] to be passed as job arguments).

All credit goes to [@jdelStrother] who [suggested this solution][2].

Fixes [hotwired#522] and [hotwired#535]

[1]: https://github.com/sidekiq/sidekiq/wiki/Best-Practices#1-make-your-job-parameters-small-and-simple
[2]: hotwired#535 (comment)
[@jdelStrother]: https://github.com/jdelStrother
[hotwired#522]: hotwired#522
[hotwired#535]: hotwired#535

app/jobs/turbo/streams/broadcast_stream_job.rb

@@ -4,4 +4,8 @@ class Turbo::Streams::BroadcastStreamJob < ActiveJob::Base
   def perform(stream, content:)
     Turbo::StreamsChannel.broadcast_stream_to(stream, content: content)
   end
+
+  def self.perform_later(stream, content:)
+    super(stream, content: content.to_str)
+  end
 end