Graylog output should send non-empty short_message

[aseev-xx]

Bug report

Graylog output plugin send empty string in short_message variable. In source code (https://github.com/influxdata/telegraf/blob/master/plugins/outputs/graylog/graylog.go#L216):
m["short_message"] = " "

This leads to error kind:

2016-11-14T18:43:00.203+03:00 DEBUG [DecodingProcessor] Dropping incomplete message RawMessage{id=058d2ca0-aa81-11e6-8b1d-fa163e114f3b, journalOffset=3910, codec=gelf, payloadSize=137, timestamp=2016-11-14T15:43:00.202Z} on input <5829c1620b85885f77706457>. Parsed fields: [{total=0, in=0, name=swap, used_percent=0, _id=058d53b0-aa81-11e6-8b1d-fa163e114f3b, source=<HOSTNAME>, used=0, message=, free=0, timestamp=2016-11-14T15:43:00.000Z, out=0}]

As workaround, i'm recompile telegraf with:
m["short_message"] = "telegraf"
and all issues decided.

Relevant telegraf.conf:

[[inputs.swap]]

[[outputs.graylog]]
   servers = ["127.0.0.1:12201"]

Steps to reproduce:

the easiest way to reproduce error is:

normal work:
bash# echo -e '{"version": "1.1","host":"example.org","short_message":"Short message","full_message":"Full_msg","level":1,"_user_id":9001,"_some_info":"foo","_some_env_var":"bar"}\0' | nc -w 1 -u 127.0.0.1 12201
not work with error dropping incomplete message:
bash# echo -e '{"version": "1.1","host":"example.org","short_message":" ","full_message":"Full_msg","level":1,"_user_id":9001,"_some_info":"foo","_some_env_var":"bar"}\0' | nc -w 1 -u 127.0.0.1 12201

Expected behavior:

Data should be correctly parsed

Actual behavior:

Data is not parsed correctly and discarded

[sparrc]

what version of graylog & telegraf are you using?

[aseev-xx]

I tested Graylog v2.1.2 & Telegraf 1.0.1