Correctly parse repeated header entries

Message headers like this are not correctly parsed:

```
MESSAGE
subscription:id
ack:3
message-id:apollo-default-d41
reply-to:/queue/temp.default.apollo-default-d4.local
reply-to:/temp-queue/local
destination:/queue/manager
content-length:333
```

According to http://stomp.github.io/stomp-specification-1.2.html#Repeated_Header_Entries the first 'reply-to' has to be used as header value.