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Added unsubscribe_contact method to Mailjet::List. Mailjet::ApiError has been updated since it didn't handle the error message in case unsubscribe_contact fails (plain string instead of array of strings).
Mailjet::Reporting test case for reportGeoip updated since api call is supposed to return an array, ref: https://eu.mailjet.com/docs/api/report/geoip.
Also, in Mailjet::Campaign test the sleeping time for waiting the campaign to be sent has been relaxed (from 10s to 20s) to avoid test failing due to delay issues.