Fix: Callbacks for process.stdout / .stderr (#2810)

* Fix callbacks for process.stdout /.stderr

The overrides for `process.stdout.write` and `process.stderr.write` did not invoke the provided callbacks when they’re done.

* Fix: write callback is an optional parameter

* Append encoding parameter to all  calls

lib/ProcessContainer.js

@@ -201,12 +201,15 @@ function exec(script, stds) {
         else
           log_data = string.toString();
 
-        stds.std && stds.std.write && stds.std.write(log_data);
+        stds.std && stds.std.write && stds.std.write(log_data, encoding);
 
         // hardcoded values of special log path to not write on disk
-        if (pm2_env.pm_err_log_path !== 'NULL' && pm2_env.pm_err_log_path !== '/dev/null') {
-          stds.err.write && stds.err.write(log_data, encoding, cb);
-        };
+        if (pm2_env.pm_err_log_path !== 'NULL' && pm2_env.pm_err_log_path !== '/dev/null' && stds.err.write) {
+          stds.err.write(log_data, encoding, cb);
+        }
+        else if (cb) {
+          cb();
+        }
 
         process.send({
           type : 'log:err',
@@ -235,11 +238,14 @@ function exec(script, stds) {
         else
           log_data = string.toString();
 
-        stds.std && stds.std.write && stds.std.write(log_data);
+        stds.std && stds.std.write && stds.std.write(log_data, encoding);
 
         // hardcoded values of special log path to not write on disk
-        if (pm2_env.pm_out_log_path !== 'NULL' && pm2_env.pm_out_log_path !== '/dev/null') {
-          stds.out.write && stds.out.write(log_data);
+        if (pm2_env.pm_out_log_path !== 'NULL' && pm2_env.pm_out_log_path !== '/dev/null' && stds.out.write) {
+          stds.out.write(log_data, encoding, cb);
+        }
+        else if (cb) {
+          cb();
         }
 
         process.send({