Add integration test to help prevent JSON loading regressions [changelog skip]

[cjlarose]

Description

This change adds an integration test for the changes in #2269, in order to prevent regressions similar to #1801.

It's important that in a puma cluster, that the master process doesn't require the json gem, because doing to makes it not possible for users to upgrade or downgrade their application using a phased restart if they change the version of the json gem they're using.

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[nateberkopec]

Great thinking! Glad we added this.