Add xmlschema to accepted Rails/TimeZone methods.

[koic]

Follow up of #5761 and #1787.

iso8601 and rfc3339 methods are aliases of xmlschema.
https://github.com/rails/rails/blob/v5.2.0/activesupport/lib/active_support/time_with_zone.rb#L145-L153

If this cop accepts these methods, this cop need to also accept the original xmlschema.

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