Readd json serialize on schema objects to simplify json encoding them

[vsouz4]

• small fix for README template, use doesn't require \ at the beginning
• fix for collections toArray docblock, correct @return is array, not array of the inner entities
• before version 3, we could use json_encode with schema objects, now, if one wants to respond these objects directly, one needs usually to check if it's empty (e.g. json_encode(['myValue' => $myObject->getMyValue() === null ? null : $myObject->getMyValue()->toArray()]), by implementing JsonSerializable again, we offer one more way to serialize the api client structures, in a backwards compatible fashion.

[mikemadisonweb]

The PR looks sane. @vsouz4 What about breaking change that @ivastly mentioned in the previous PR? With array of arrays instead of an array of entities? Is it valid? Unfortunately, I don't have a real client now to test it.

[vsouz4]

The PR looks sane. @vsouz4 What about breaking change that @ivastly mentioned in the previous PR? With array of arrays instead of an array of entities? Is it valid? Unfortunately, I don't have a real client now to test it.

I've spoken with @ivastly seems like he had some code like

array_map(
    static function (SomeObject ...) {
        ...
    },
    $someCollection->toArray()
)

and had to change ->toArray() to $someCollection->getIterator()->getArrayCopy() or iterator_to_array($someCollection). Turns out the change I've introduced on that previous PR (3.0.3) should've been made together with the major version 3.0.0 update, because with the lack of jsonSerialize, we had to start using toArray to serialize our objects, and we needed toArray to call inner toArray's as well otherwise we would need to recursively/manually serialize these inner objects.