Fix: make jsonSerialize backward compatibile with PHP 7.4

[albertoarena]

Issue

• Fractal::jsonSerialize() return type was changed to mixed by commit 4d3b8a6#diff-9ebff18d48644c7d6e2c1006245d57ef9028099e796f76f967c8656aedadcf4bR458

  fractalistic/src/Fractal.php

  Line 458 in 5a89b24

  public function jsonSerialize(): mixed

• mixed pseudo type is not compatible with PHP 7.4

• a type error is triggered

  TypeError
  Return value of Spatie\Fractalistic\Fractal::jsonSerialize() must be an instance of Spatie\Fractalistic\mixed, array returned 
  

Solution

• make return type compatible with PHP 7.4
• add attribute #[\ReturnTypeWillChange] to prevent warnings in PHP 8.1

Tests

• run successfully with PHP 7.4 and 8.1

Notes

• This fixes issue Fractal::jsonSerialize() triggers a Type error (PHP 7.4.24) #54

[freekmurze]

Thanks!