[PHP] Fix deserialize ApiException as a Model

[ymilin]

PR checklist

• [x] Read the contribution guidelines.
• [x] Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

When defining default responses (ApiException) as a Schema Object in Swagger declaration files, the current implementation cannot deserialize the Model properly into ApiException::$responseObject.

Added a check for $returnType and json_decode appropriately (same logic as the try part of same function for success responses)

fix #7544

[ymilin]

@jebentier @dkarlovi @mandrean @jfastnacht @ackintosh

[ackintosh]

Thanks for the PR! I'll check it later. 😉