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
Mimic: correct files mode
Merge pull request #156 from krasnoukhov/better-tests

Further tests improvements
Parse strings with NULL bytes
Throw parse error if string contains NULL byte
Set end pointers for SC parse
Extract input string assignment to separate function
Safer conditions to detect buffer end