Fix iteration of queue messages

[ycombinator]

The motivation for this PR is https://community.rackspace.com/developers/f/7/p/4494/11318#11318.

This PR introduces a custom iterator (OpenCloud\Queues\Collection\MessageIterator) for iterating over the results of $queue->listMessages(). This is necessary because the underlying HTTP API's response, unlike other paginated list API responses, does not expose the marker property in each item (message) in the list of items (messages) returned. The common PaginatedIterator class relies on this API idiom and, therefore, cannot be used as-is for this particular API.

[jamiehannaford]

We can probably apply these changes to a new QueuesIterator class that would live in the OpenCloud\Queues\Collection namespace. Your new iterator would extend the base PaginatedIterator and override the shouldAppend method, in a similar way to how DnsIterator does it.

This will isolate the changes to just this service and mitigate the risk of other things breaking. If this only applies to a single operation (like ListMessages), you could restrict the scope of the new class to this operation only - and have all the other Queues collection operations go through the base class.

[ycombinator]

@jamiehannaford I've taken your advice and created a custom iterator. This PR is ready for review now. Thank you!

[ycombinator]

What do you think about making them even more strict by throwing an exception if $numMessagesCreated != $numMessagesListed?

Yes, of course. I'll see if I can pull in phpunit's assertion framework and use it for this.