fails fluentd with RUBYOPT with two or more options

[tdtds]

Describe the bug
When RUBYOPT environment variable has two or more options (e.g. "-rfoo -rbar"), fluentd fails to start supervisor and into infinite loop.

To Reproduce

i% RUBYOPT="-rbundler/setup -rtest-unit" fluentd                                                                     [basic_input_plugin]
2020-01-28 11:49:33 +0900 [info]: parsing config file is succeeded path="/etc/fluent/fluent.conf"
2020-01-28 11:49:33 +0900 [info]: gem 'fluentd' version '1.9.0'
2020-01-28 11:49:33 +0900 [info]: gem 'fluent-plugin-wmi' version '0.1.0'
2020-01-28 11:49:33 +0900 [info]: using configuration file: <ROOT>
</ROOT>
2020-01-28 11:49:33 +0900 [info]: starting fluentd-1.9.0 pid=32730 ruby="2.5.7"
2020-01-28 11:49:33 +0900 [info]: spawn command to main:  cmdline=["/home/sho/.rbenv/versions/2.5.7/bin/ruby", "-Eascii-8bit:ascii-8bit", "-rbundler/setup -rtest-unit", "/home/sho/.rbenv/versions/2.5/bin/fluentd", "--under-supervisor"]
Traceback (most recent call last):
        1: from /home/sho/.rbenv/versions/2.5.7/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require'
/home/sho/.rbenv/versions/2.5.7/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require': cannot load such file -- bundler/setup -rtest-unit (LoadError)
2020-01-28 11:49:33 +0900 [info]: Worker 0 finished unexpectedly with status 1
:
:
(repeat infinite...)

Expected behavior
fluentd stops with clearly error messages.

Your Environment

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