Fix Sidekiq setting non-serializable Class as resource

[delner]

Fixes #943

In Rails ActiveJob via rails/rails@0e64348 they pass wrapped as a Class constant instead of a String. When this constant is set as the resource, it's suspected that it cannot be converted via Msgpack and raises an error on serialization.

This pull request makes sure the job resource is always a string.

[delner]

I haven't been able to use tests to recreate the original bug, so the current implementation is a bit of a shot in the dark.

Whenever I get job in the instrumentation, it appears all the keys have already been serialized to strings.... I'm not sure if this is a side effect of using Sidekiq::Testing.inline! or not.

{"retry"=>true,
 "queue"=>"default",
 "class"=>"ActiveJob::QueueAdapters::SidekiqAdapter::JobWrapper",
 "wrapped"=>"EmptyJob",
 "args"=>
  [{"job_class"=>"EmptyJob",
    "job_id"=>"72cf8cb1-f9e4-4d3c-94d7-7a90ed69d987",
    "provider_job_id"=>nil,
    "queue_name"=>"default",
    "priority"=>nil,
    "arguments"=>[],
    "executions"=>0,
    "exception_executions"=>{},
    "locale"=>"en",
    "timezone"=>"UTC",
    "enqueued_at"=>"2020-02-13T19:43:22Z"}],
 "jid"=>"ac04eb720a1771775a72dca7",
 "created_at"=>1581623002.2207384,
 "id"=>"4edf20b05f11e86a08d4b3d0"}

@pj0tr can you confirm this fix actually works?

[marcotc]

!

[delner]

Reported as working via #943 (comment) so I think this is good to merge.