[PHP] Bugfix: DateTime object on query

[thomasphansen]

This PR fixes bug #7712. PR #11225 was a major step to fix query params support (thanks, @nadar! 😄), but it left out DateTime objects (which were supported previously, through the ::toString() method).

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[wing328]

LGTM.

[nadar]

Sorry for the introduced break. Unit Tests FTW :-)