Fix oj errors

See #656.

Relaxes `oj`'s dump mode to `:compat`, centralizes all JSON (de-)serialization, uses different compatibility modes for load and dump (following the example of `multi_json`'s `oj` adapter) and removes any explicit `.to_json` calls (and `mimic_json` from `oj`) for the avoidance of doubt.
is this custom to_json not required?