title: Nodejs event in promis doesnt resolve
body: <p>When I call <code>pdfCreate</code>, an incident is loaded, which works fine. </p>
  
  <p>Then <code>createIncidentReport</code> is called from an included service, this works fine to.</p>
  
  <p><code>pdfStream.on('finish')</code> is called once the PDF is written to the disk, this works fine on the console. The log is written, but then the mailservice isn't called. How should I resolve the promise?</p>
  
  <pre><code>function pdfCreate(incidentId) {
          Incident.findById(incidentId)
              .then(incident =&gt; pdfService.createIncidentReport(incident))
              .then(() =&gt; mailservice.sendPdfCreatedMail(incident));
      }   
  
       function createIncidentReport(incident, res) {
              return new Promise((resolve, reject) =&gt; {
                       let pdfStream = fs.createWriteStream(settings.PDF.DIRECTORY + incident.id + '.pdf');
                       incidentReport.pipe(pdfStream);
                       pdfStream.on('finish', function () {
                           console.log('finished writing pdf')
                           resolve();
                       });
              }
          }
  
      function sendPdfCreatedMail(incident) {
          console.log('mail');
          let message = {
              to: incident.reporter.email,
              subject: 'Incident completed',
              text: 'Incident has been completed.',
              html: 'Incident has been completed.'
          };
  
          sendMessage(message);
  
      }
  </code></pre>