Fix thread variable in timezone

What does this MR do and why?

Related to #48 (closed), the Time.zone is a thread variable, we should change to config the instance variable Time.zone_default.

Refer to:

1. https://stackoverflow.com/questions/38327283/why-does-time-zone-get-set-to-nil-inside-thread
2. https://github.com/rails/rails/blob/v7.0.2.4/activesupport/lib/active_support/core_ext/time/zones.rb#L15

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