```ruby
require_relative '../../lib/sub_diff/diff_collection'

RSpec.describe SubDiff::DiffCollection do
  subject { described_class.new(diffs_with_empty) }

  let(:diffs_with_empty) do
    diffs.dup << diff('')
  end

  let(:diffs) do
    [diff('one '), diff('two'), diff(' three ')]
  end

  def diff(value)
    double('diff', changed?: false, empty?: value.empty?, to_s: value)
  end

  describe '#changed?' do
    it 'should return true if any diffs have changed' do
      expect(diffs.first).to receive(:changed?).and_return(true)
      expect(subject).to be_changed
    end

    it 'should return false if no diffs have changed' do
      expect(subject).not_to be_changed
    end
  end

  describe '#diffs' do
    it 'should exclude empty diffs' do
      expect(subject.diffs).to eq diffs
    end
  end

  describe '#each' do
    it { is_expected.to be_an Enumerable }
    it { is_expected.to delegate(:each).to(:diffs) }
  end

  describe '#size' do
    it { is_expected.to delegate(:size).to(:diffs) }

    it 'should not use the string size' do
      expect(subject.size).not_to eq subject.to_s.size
    end
  end

  describe '#to_s' do
    it 'should join the diff values' do
      expect(subject.to_s).to eq 'one two three '
    end
  end
end
```

Add `to_str` double mock for `Diff` objects
user
```ruby
require_relative '../../lib/sub_diff/diff_collection'

RSpec.describe SubDiff::DiffCollection do
  subject { described_class.new(diffs_with_empty) }

  let(:diffs_with_empty) do
    diffs.dup << diff('')
  end

  let(:diffs) do
    [diff('one '), diff('two'), diff(' three ')]
  end

  def diff(value)
    double 'diff', changed?: false,
                   empty?: value.empty?,
                   to_s: value,
                   to_str: value
  end

  describe '#changed?' do
    it 'should return true if any diffs have changed' do
      expect(diffs.first).to receive(:changed?).and_return(true)
      expect(subject).to be_changed
    end

    it 'should return false if no diffs have changed' do
      expect(subject).not_to be_changed
    end
  end

  describe '#diffs' do
    it 'should exclude empty diffs' do
      expect(subject.diffs).to eq diffs
    end
  end

  describe '#each' do
    it { is_expected.to be_an Enumerable }
    it { is_expected.to delegate(:each).to(:diffs) }
  end

  describe '#size' do
    it { is_expected.to delegate(:size).to(:diffs) }

    it 'should not use the string size' do
      expect(subject.size).not_to eq subject.to_s.size
    end
  end

  describe '#to_s' do
    it 'should join the diff values' do
      expect(subject.to_s).to eq 'one two three '
    end
  end
end
```
assistant