handle data that have JSON-friendly representation

When converting event data into the JSON payload to send to BQ, have
special handling for Hash objects. However, some data will come in as on
object that isn't immediately convertable to something JSON-friendly,
such as models (e.g. StoreModel or ActiveModel objects).

This change converts everything to a more standard JSON-friendly
representation before processing for sending to BQ, meaning that models
are converted to a Hash object which we can then to_json as needed.

lib/dfe/analytics/event.rb

@@ -101,6 +101,8 @@ def with_request_uuid(request_id)
 
       def hash_to_kv_pairs(hash)
         hash.map do |(key, value)|
+          value = value.try(:as_json)
+
           if value.in? [true, false]
             value = value.to_s
           elsif value.is_a?(Hash)

spec/dfe/analytics/event_spec.rb

@@ -62,6 +62,17 @@
   end
 
   describe 'data pairs' do
+    let(:has_as_json_class) do
+      Struct.new(:colour, :is_cat) do
+        def as_json
+          {
+            colour: colour,
+            is_cat: is_cat
+          }
+        end
+      end
+    end
+
     it 'converts booleans to strings' do
       event = described_class.new
       output = event.with_data(key: true).as_json
@@ -73,6 +84,12 @@
       output = event.with_data(key: { equality_and_diversity: { ethnic_background: 'Irish' } }).as_json
       expect(output['data'].first['value']).to eq ['{"equality_and_diversity":{"ethnic_background":"Irish"}}']
     end
+
+    it 'handles objects that have JSON-friendly structures' do
+      event = described_class.new
+      output = event.with_data(as_json_object: has_as_json_class.new(:green, true)).as_json
+      expect(output['data'].first['value']).to eq ['{"colour":"green","is_cat":true}']
+    end
   end
 
   def fake_request(overrides = {})