Forwarded tag is violating the specification

[okumin]

Check CONTRIBUTING guideline first and here is the list to help us investigate the problem.

• fluentd 0.14.11
• CentOS 7.2
• configuration

Fluentd 0.14 seems to encode tags into msgpack's bin format on forwarding.
But The specification says that tags should be str format and 0.12 is encoding tags into str.

I found this problem by the following process and packet capturing(but lost logs……).
https://github.com/okumin/influent-benchmark