Improve $LOADED_FEATURES append pattern

```start_with?('json')``` is not correct implementation.
For example, this prevents ```json_spec``` gem from being loaded.
Regular expression which checks if there is a file or folder starting
with json works better.
Improve $LOADED_FEATURES: fix merge hiccup
Improve $LOADED_FEATURES: do not mess with return
Merge pull request #157 from krasnoukhov/loaded-features

Improve $LOADED_FEATURES append pattern
Further tests improvements

* Extract common mimic tests to isolated/shared
* Get rid from minitest-around
* Introduce rails_after and rails_before mimic tests
* Give tests proper names
  * MimicAlone – when json is never required
  * MimicDefine – simple test to check if JSON is properly defined
Mimic: fix everything around me

* Define JSON::State as empty class to be compatible with ActiveSupport
* Move Object#to_json definition to a non-verbose block
* Fix test_object failing test

Dolla-dolla bill y'all