Add support for if-unused and if-empty for delete_queue

[shashankmehra]

From HTTP API docs

When DELETEing a queue you can add the query string parameters if-empty=true and / or if-unused=true. These prevent the delete from succeeding if the queue contains messages, or has consumers, respectively.

This PR adds supports for providing these query parameters and the tests to confirm the behaviour.

[michaelklishin]

According to the diff, this PR does modify some existing tests and that's it.

[michaelklishin]

Hm, now I see more recent changes, not sure what happened to the Files changes tab.

[michaelklishin]

Thank you!