Override ip_address JSON representation for AuditEvents

When called via as_json, AuditEvents serialize
ip_addresses as a Hash with 3 parts as opposed
to a human readable string.

This caused unexpected breaking changes to the
audit log JSON output.

app/models/audit_event.rb

@@ -58,6 +58,12 @@ class AuditEvent < ApplicationRecord
     end
   end
 
+  def as_json(options = {})
+    super(options).tap do |json|
+      json['ip_address'] = self.ip_address.to_s
+    end
+  end
+
   private
 
   def default_author_value

spec/models/audit_event_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe AuditEvent do
+  let_it_be(:audit_event) { create(:project_audit_event) }
+  subject { audit_event }
+
+  describe '#as_json' do
+    context 'ip_address' do
+      subject { build(:group_audit_event, ip_address: '192.168.1.1').as_json }
+
+      it 'overrides the ip_address with its string value' do
+        expect(subject['ip_address']).to eq('192.168.1.1')
+      end
+    end
+  end
+end