[php] Set $multipart to true if content-type is multipart/form-data (#5536)

[simonhammes]

The PHP client does not correctly identify multipart/form-data requests without files. See #5536 (comment) for details.
This PR fixes this bug.

Related: #5536

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[simonhammes]

TC mentions: @jebentier @dkarlovi @mandrean @jfastnacht @ybelenko @renepardon

[simonhammes]

What I did not manage to figure out: Is it possible to move this check to generation time?

[wing328]

thanks for the PR

https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java#L4610

what about using isMultipart in codegen operation?