Base.mode is supports thread local variable assignment

[lancecarlson]

[lancecarlson]

This is only supported in Rails 5+. Perhaps it makes sense to start supporting 5+ at some point?

[lancecarlson]

I can add the active support file into the active merchant and require it if the rails version is older. Is this desired?

[github-actions[bot]]

To provide a cleaner slate for the maintenance of the library, this PR/Issue is being labeled stale after 60 days without activity. It will be closed in 14 days unless you comment with an update regarding its applicability to the current build. Thank you!