Fix request storing when RequestLogExpirationDuration is set [bug]

[pbenko-xitaso]

References

Fixes #1454

ResponseMessageBuilder now properly sets the the Response.DateTime instead of leaving it on its default, which caused the immediate deletion of the Request downstream, when evaluating Requests to purge.

The ResponseMessageBuilder isn't static anymore, to enable testing without propagating a DateTimeUtility throughout the codebase.

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[StefH]

@pbenko-xitaso
Can you take a look at my comments?

[pbenko-xitaso]

Sorry, this PR was the last thing I did before falling sick last week.

Done.