Support JSON serialization in Hanami::DB::Struct

We would like structs in Hanami to support simple JSON serialization
following the `#to_json` convention.

There have been discussions of this functionality in both ROM and
Dry-Struct projects, where this was rejected as not appropriate for
those gems:

- dry-rb/dry-struct#166
- rom-rb/rom#403

This adds a simple `#to_json` method to `Hanami::DB::Struct` to support
the simple case for serialization.

lib/hanami/db/struct.rb

@@ -5,6 +5,14 @@ module DB
     # @api public
     # @since 2.2.0
     class Struct < ROM::Struct
+      # @api public
+      # @since 2.2.0
+      #
+      # Simple conversion of attributes to JSON format, without this method, the instance of the struct gets converted
+      # to a string
+      def to_json(*args)
+        to_h.to_json(*args)
+      end
     end
   end
 end

spec/unit/struct_spec.rb

@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+RSpec.describe Hanami::DB::Struct do
+  let!(:config) do
+    ROM::Configuration.new(:sql, "sqlite::memory") do |conf|
+      conf.default.create_table(:users) do
+        primary_key :id
+        column :first_name, String
+        column :last_name, String
+      end
+
+      conf.relation(:users) do
+        schema(infer: true)
+      end
+    end
+  end
+
+  let(:rom) { ROM.container(config) }
+
+  before do
+    module Test
+      module Entities
+        class User < Hanami::DB::Struct
+          def full_name
+            "#{first_name} #{last_name}"
+          end
+        end
+      end
+
+      class UserRepo < ROM::Repository[:users]
+        struct_namespace Entities
+      end
+    end
+  end
+
+  describe "#to_json" do
+    before do
+      rom.relations[:users].changeset(:create, id: 1, first_name: "L", last_name: "L").commit
+    end
+
+    it "converts to json" do
+      repo = Test::UserRepo.new(rom)
+      struct = repo.users.by_pk(1).one!
+      expect(struct.full_name).to eq "L L"
+      expect(struct.to_json).to eq "{\"id\":1,\"first_name\":\"L\",\"last_name\":\"L\"}"
+    end
+  end
+end