User policy with overflow "reject-publish" does not revert to default value when overflow removed from policy.

[kevinpayne]

This is a small issue I found when using overflow with user policies in RabbitMQ 3.7.4

1. Create a queue user policy with overflow value of reject-publish . Set max-length to 100.
2. Verify producer receives nacks once downstream queue limit (100) has been reached. This works as expected.
3. Now update the same policy to remove the overflow property altogether. The expectation is that overflow will revert to the default value drop-head and no longer send nacks to the producer.
4. Producer sends a message to the full queue and unexpectedly still receives nacks.

[lukebakken]

Fixed by #1994