failed fluentd with RUBYOPT has "-E" option

[tdtds]

Describe the bug
When RUBYOPT environment variable has "-E" option, fluentd fails to start supervisor and into infinite loop.

To Reproduce

% RUBYOPT=-Eutf-8 fluentd                                                                                           [basic_input_plugin]
2020-01-28 11:35:17 +0900 [info]: parsing config file is succeeded path="/etc/fluent/fluent.conf"
2020-01-28 11:35:17 +0900 [info]: gem 'fluentd' version '1.9.0'
2020-01-28 11:35:17 +0900 [info]: using configuration file: <ROOT>
</ROOT>
2020-01-28 11:35:17 +0900 [info]: starting fluentd-1.9.0 pid=31909 ruby="2.5.7"
2020-01-28 11:35:17 +0900 [info]: spawn command to main:  cmdline=["/home/sho/.rbenv/versions/2.5.7/bin/ruby", "-Eascii-8bit:ascii-8bit", "-Eutf-8", "/home/sho/.rbenv/versions/2.5/bin/fluentd", "--under-supervisor"]
Traceback (most recent call last):
/home/sho/.rbenv/versions/2.5.7/bin/ruby: default_external already set to ascii-8bit (RuntimeError)
2020-01-28 11:35:17 +0900 [info]: Worker 0 finished unexpectedly with status 1
:
:
(repeat infinite...)

Expected behavior
fluentd stops with clearly error messages.
Or fluentd runs without "-Eascii-8bit:ascii-8bit" is better.

Your Environment

• fluentd 1.9.0
• ruby 2.5.7p206
• Debian GNU/Linux 9 (on Windows WSL)