HornetQTextMessage sets payload as SimpleString

When [HornetQTextMessage](https://github.com/hornetq/hornetq/blob/master/hornetq-jms-client/src/main/java/org/hornetq/jms/client/HornetQTextMessage.java) is created, method <code>setText</code> is used to set its payload. Internally it uses <code>writeNullableSimpleString</code>, when writing it to the buffer.

In case the message is sent to <code>OutgoingVertxEventHandler</code> with <code>VertxMessageType</code> set to <code>TYPE_STRING</code> we have to invoke <code>readNullableSimpleString</code> instead of <code>readString</code>, otherwise it will not be able to decode it properly, failing with an exception.