Message timestamp not included in delivery report

**Environment Information**
- OS [e.g. Mac, Arch, Windows 10]: Mac
- Node Version [e.g. 8.2.1]: 8.9.4
- NPM Version [e.g. 5.4.2]: 5.6
- C++ Toolchain [e.g. Visual Studio, llvm, g++]:
- node-rdkafka version [e.g. 2.3.3]: 2.3.3

**Steps to Reproduce**

Produce a message and include a dr_cb function. The report does not include the produced message timestamp.

**node-rdkafka Configuration Settings**
...
'request.required.acks': 1,
'dr_cb': function(err, data) {
if( data.value ) {
data.value = data.value.toString('utf8')
}
delivery_report.onNext(data)
}

**Additional context**

The timestamp is available to consumers, it's just not in the delivery report.

Start by tracing the dr_cb delivery-report path described in the reproduction and compare its data with the timestamp available to consumers. Confirm the expected timestamp behavior in the node-rdkafka bindings and verify that a produced message's delivery report includes it.

Written by the indexing model from the issue text.