set encoding when reading from streams

using .toString() works most of the time but can lead to utf8 characters being
broken across chunk boundaries.

https://nodejs.org/api/stream.html#stream_readable_setencoding_encoding

app/js/OutputFileOptimiser.js

@@ -78,7 +78,7 @@ module.exports = OutputFileOptimiser = {
     const timer = new Metrics.Timer('qpdf')
     const proc = spawn('qpdf', args)
     let stdout = ''
-    proc.stdout.on('data', chunk => (stdout += chunk.toString()))
+    proc.stdout.setEncoding('utf8').on('data', chunk => (stdout += chunk))
     callback = _.once(callback) // avoid double call back for error and close event
     proc.on('error', function(err) {
       logger.warn({ err, args }, 'qpdf failed')