Ensure `#custom_json` is JSON, not Ruby object
Ensure `#custom_json` is JSON, not Ruby object

This way, an actual JSON string can be given as `custom_json`
parameter.
Otherwise, when an actual JSON string is given via the parameter,
we turn *that* into a string.

computology/dpl,testfairy/dpl,travis-ci/dpl,travis-ci/dpl,travis-ci/dpl,testfairy/dpl,testfairy/dpl