Fix(serializer): Properly normalize `DateTimeInterface` objects

[bpolaszek]

Pull Request

Related issue

Fixes #304

What does this PR do?

• DateTimeInterface objects are now normalized as unix timestamps integers, in the context of a serialization initiated by this bundle.

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• [x] Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
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Thank you so much for contributing to Meilisearch!

[bpolaszek]

FYI, I could not run PHPUnit on my side - waiting for #308 to be merged

[norkunas]

Not sure how we should tackle compat with 5.4

[bpolaszek]

Not sure how we should tackle compat with 5.4

I think the mixed keyword should be avoided, let me give it a try

[norkunas]

bors merge

[norkunas]

bors ping

[meili-bors[bot]]

pong

[meili-bors[bot]]

Timed out.

[norkunas]

bors try

[meili-bors[bot]]

try

Timed out.

[norkunas]

bors try

[meili-bors[bot]]

try

Timed out.

[norkunas]

Thank you @bpolaszek