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/CompileManager.js

@@ -334,7 +334,7 @@ module.exports = CompileManager = {
       proc.on('error', callback)
 
       let stderr = ''
-      proc.stderr.on('data', chunk => (stderr += chunk.toString()))
+      proc.stderr.setEncoding('utf8').on('data', chunk => (stderr += chunk))
 
       return proc.on('close', function(code) {
         if (code === 0) {

app/js/LocalCommandRunner.js

@@ -46,7 +46,7 @@ module.exports = CommandRunner = {
     const proc = spawn(command[0], command.slice(1), { cwd: directory, env })
 
     let stdout = ''
-    proc.stdout.on('data', data => (stdout += data))
+    proc.stdout.setEncoding('utf8').on('data', data => (stdout += data))
 
     proc.on('error', function(err) {
       logger.err(

app/js/OutputFileFinder.js

@@ -87,7 +87,7 @@ module.exports = OutputFileFinder = {
 
     const proc = spawn('find', args)
     let stdout = ''
-    proc.stdout.on('data', chunk => (stdout += chunk.toString()))
+    proc.stdout.setEncoding('utf8').on('data', chunk => (stdout += chunk))
     proc.on('error', callback)
     return proc.on('close', function(code) {
       if (code !== 0) {

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')