Bad publishTime format used when publishTime not provided in the message context

[TonyVlcek]

Current format:

gmdate('%Y-%m-%dT%H:%M:%S.%6NZ')

That produces, for example, this string %2023-%09-%29GMT%13:%Sep:%th.%650 instead of the RFC 3339 format.

It's an easy fix; I'll send a Pull Request. This might also be relevant for #66

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I came across this when testing my cloud function with the PubSub emulator. There was a similar issue in the node implementation: googleapis/google-cloud-node#1925.

Ultimately, it would be great if the emulator included this property. Is there a right place to file an issue with the PubSub emulator?
I assume that this is beyond the scope of this PHP repo, nonetheless, from my tests, it seems like there are situations where the publishTime is present in the PubSub message (for example when I'm pulling the message via the PubSub API), but not present when the payload reaches the cloud functions framework.