[php-next-gen] fix handling with enums in object serializer

[reinfi]

I am unsure if the whole check could be removed because in the setter method of the model there is a phpdoc typehint which states that only an enum is allowed. But as long as there are no native type hints it could not be ensured that the value is always an enum.

$data in this case should never be overwritten because it contains the OpenApi information.

In my opinion serialization of the value should always be a scalar value because otherwise receiving clients must understand PHP enums serialization.

Offtopic: there are some more issues with enums, i.e. in query parameters. But I will fix them with separate pull requests so they are clean and simple.

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[reinfi]

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

Still feels awkward to mention so many people :-D

[wing328]

@reinfi thanks for the PR, which has been merged into the master

Can you please PM me via Slack when you've time?

https://join.slack.com/t/openapi-generator/shared_invite/zt-12jxxd7p2-XUeQM~4pzsU9x~eGLQqX2g

[wing328]

FYI. I've created a Slack channel (https://app.slack.com/client/TLQFRCNJZ/C0600D9TMK9) to facilitate discussions related to the PHP generators