Improve mimic of JSON::ParserError

* Do const_set on Ruby level instead of C
* Extract common mimic tests to shared_mimic
* Make sure before/after tests are working
* Improve mimic_single test

As a result of this Rails integration is working properly.
The only caveeat is that Oj.mimic_JSON should be called after
json is loaded for this case. Basically this means that
it should be called after Rails is loaded.