≥2.9.1: stack overflow when serializing a class

[jamesarosen]

We have some .jbuilder files that look like

id   thing.id
type thing.class
…

Ever since e2e8623, this results in a stack overflow. It's easy to solve with

id   thing.id
type thing.class.name
…

I don't know whether the maintainers consider this a regression. If so, this might be difficult to catch since the Class instance could be passed to Jbuilder in any number of ways.