DEV: default Oj to compat mode

Out-of-the-box Oj uses :object mode, this shifts us to use :compat mode
by default which is safer.
It means any de-serialization going forward will default to this mode.

If we wish to serialize or deserialize arbitrary objects going forward with
no json interfaces we will have to opt in.

config/initializers/100-oj.rb

@@ -3,6 +3,7 @@
 Oj::Rails.set_encoder()
 Oj::Rails.set_decoder()
 Oj::Rails.optimize()
+Oj.default_options = Oj.default_options.merge(mode: :compat)
 
 # Not sure why it's not using this by default!
 MultiJson.engine = :oj