[PHP] ObjectSerializer::deserialize() associative arrays bugfix

Created by: thomasphansen

This comes from a bug we found while dealing with the Tripletex API.

ObjectSerializer::deserialize()'s last else block instantiates an object of type $class and tries to populate it with the contents of $data. From this point on, $data is always handled as an object, as we check for its properties ($data->{$discriminator} or $data->{$instance::attributeMap()[$property]}.

But while working with Tripletex (and after a lot of debugging), we figured out that in some very specific cases the received $data parameter was an associative array, which then caused all properties from the instance to become empty.

This PR casts $data to object, in case it is an array. It is absolutely harmless, since it happens in the last else block (where we only care for an object anyway). It's only purpose is to make sure that the data in a received associative array will be properly handled as an object.

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