[php][bug] Fix generic ObjectSerializer error when deserializing SomeClass[] data

[alsma]

json_decode without 2nd parameter will create \stdClass object for array/associative array, so exception Invalid array '$class' is always thrown

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@jebentier (2017/07), @dkarlovi (2017/07), @mandrean (2017/08), @jfastnacht (2017/09), @ackintosh (2017/09) ❤️, @ybelenko (2018/07), @renepardon (2018/12)

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