Add spec for :head option
user
```ruby
# encoding: utf-8

RSpec.describe TTY::ProgressBar, ':head' do
  let(:output) { StringIO.new('', 'w+')}

  it "animates head" do
    progress = TTY::ProgressBar.new("[:bar]", output: output, head: '>', total: 5)
    5.times { progress.advance }
    output.rewind
    expect(output.read).to eq([
      "\e[1G[>    ]",
      "\e[1G[=>   ]",
      "\e[1G[==>  ]",
      "\e[1G[===> ]",
      "\e[1G[====>]\n"
    ].join)
  end
end
```
assistant