Add `delete_exchange` method and fix Rspec 3 errors

Add `delete_exchange` method and its accompanying tests, modelled on the `delete_queue` method.

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Also, fix errors when running tests with Rspec 2.99.0.beta2 and above. See the failures in [Travis](https://travis-ci.org/ruby-amqp/rabbitmq_http_api_client/jobs/28469599).