[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

[wing328]

@simonhammes thanks for the PR, which has been merged.

Sorry for taking a bit long to review.

[simonhammes]

@wing328 Thanks for merging!

Sorry for taking a bit long to review.

No worries 🙂