Add test for encoding JsonSerializable with pre-processing

[Tigrov]

See section About jsonSerialize() at #41

[what-the-diff]

PR Summary

• Improvements to the JsonTest.php
  The JsonTest.php file has been improved for better functionality. Key modifications are made to test how objects get translated or "encoded" into JSON format.

• Inclusion of the ArrayObject class
  The ArrayObject class has been included via a use statement. This class allows arrays to behave like objects which helps in advanced operations.

• Changes to the 'testEncodeObjectProperties' method
  The testEncodeObjectProperties method was fine-tuned to only check ('assert') the result when the object Properties is converted ('encoded') to JSON.

• Addition of 'testEncodeJsonSerializableWithPreprocessing' method
  A new method called testEncodeJsonSerializableWithPreprocessing was added. This new method checks the result when an array with the elements 1, 2, and 3, put inside a Data object, is converted to JSON format.