def update_contact(contact)
      raise "contact_id or contact_number is required for updating contacts" if contact.contact_id.nil? and contact.contact_number.nil?
      save_contact(contact)
    end
def update_contact(contact)
      raise "contact_id or contact_number is required for updating contacts" if contact.contact_id.nil? and contact.contact_number.nil?
      save_contact(contact)
    end
Updates an existing Xero contact

 Usage :

 contact = xero_gateway.get_contact(some_contact_id)
 contact.email = "a_new_email_ddress"

 xero_gateway.update_contact(contact)