Fix metadata for Sidekiq strict_args!

[craigmcn]

• update scheduled_at metadata to string key, hash rocket syntax
• update spec to expect string key, hash rocket syntax
• fixes issue When include_metadata: true, Sidekiq.strict_args! fails with "Job arguments must be native JSON types" #402

[marcelolx]

Thank you @craigmcn!

[yeonhoyoon]

This could potentially be a breaking change for some applications, if scheduled_at was being accessed by symbol key.
It might have been a better idea to put this into a minor version, not patch version.

For people visiting this issue after updating the gem to 4.0.3:
metadata[:scheduled_at] should be changed to metadata["scheduled_at"]

[marcelolx]

@yeonhoyoon could you give an example of why you're accessing the scheduled_at?