add :force option to cache!

[bobdutch]

Before change 65dfb33 you could pass the :force option to cache! This worked because cache! used Rails.cache.fetch which takes the force option. That commit rewrote cache to use Rails.cache.read and Rails.cache.write which don't do anything with the force option. I use :force extensively in my jbuilder template to send a parameter that will clear and write the cache for the next request. I think this is a worthy feature to have even if it was an undocumented and unintentional feature of jbuilder pre-2.6.0
Closes #424

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @pixeltrix (or someone else) soon.

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[HarlemSquirrel]

@pixeltrix it would be awesome if you could spare a moment to review this. Thank you in advance!

[akodkod]

+1 for this

[ghost]

+1