Add test of field comparer
user
```ruby
RSpec.describe Indexer::GovukIndexFieldComparer do
  it "identifies unchanged content" do
    is_same = described_class.new.call("/some/id", "title", "some text", "some text")
    expect(is_same).to be true
  end

  it "identifies unchanged arrays of content" do
    is_same = described_class.new.call(
      "/some/id",
      "title",
      %w(value1 value2 value3),
      %w(value1 value2 value3)
    )
    expect(is_same).to be true
  end

  it "identifies changed content" do
    is_same = described_class.new.call("/some/id", "title", "some text", "other text")
    expect(is_same).to be false
  end

  it "identifies changes in arrays" do
    is_same = described_class.new.call(
      "/some/id",
      "title",
      %w(value1 value2 value3),
      %w(value1 other_value2 value3)
    )
    expect(is_same).to be false
  end

  it "identifies items added to arrays as changed" do
    is_same = described_class.new.call(
      "/some/id",
      "title",
      %w(value1 value2),
      %w(value1 value2 value3)
    )
    expect(is_same).to be false
  end

  it "identifies items removed from arrays as changed" do
    is_same = described_class.new.call(
      "/some/id",
      "title",
      %w(value1 value2 value3),
      %w(value1 value2)
    )
    expect(is_same).to be false
  end

  it "identifies removed content" do
    is_same = described_class.new.call("/some/id", "title", "some text", nil)
    expect(is_same).to be false
  end

  # We don't mind if the new search index includes additional fields
  it "ignores added content" do
    is_same = described_class.new.call("/some/id", "title", nil, "some text")
    expect(is_same).to be true
  end
end
```
assistant