Messages not appearing in the output channel

#### Expected behavior

```
reader, errorCreatingReader := client.CreateReader(pulsar.ReaderOptions{
Topic: createTopicString(namespace, topic),
StartMessageID: pulsar.LatestMessageID(),
MessageChannel: messageChannel,
ReceiverQueueSize: 10,
})
```
In this scenario, `messageChannel` gets messages in the channel whenever they arrive from the broker.
#### Actual behavior
Messages do not appear in the channel.
Note that, if I issue reader.Next(), instead of setting MessageChannel, messages do appear in the channel.

Start at client.CreateReader and trace how ReaderOptions.MessageChannel is handled, comparing that path with reader.Next(), which is reported to deliver messages. Reproduce the example with StartMessageID, MessageChannel, and ReceiverQueueSize 10, then verify that arriving broker messages appear on messageChannel without calling reader.Next().

Written by the indexing model from the issue text.