Specify explicit separator not to be affected by $;

[nobu]

Description:

Get rid to be affected by $;.

What was the end-user or developer problem that led to this PR?

Usually nothing.
When using rubygems as a library, the global variable can be set non-nil value.

What is your fix for the problem, implemented in this PR?

Specify separator argument to String#split explicitly.

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Tasks:

• Describe the problem / feature
• Write tests
• Write code to solve the problem
• Get code review from coworkers / friends

I will abide by the code of conduct.

[olleolleolle]

This avoids an issue which could be tricky to debug for a user. LGTM!

[deivid-rodriguez]

Thanks @nobu!!