SitePrism::Page#load can accept HTML.

This way we can use the same Page objects for both
integration and view specs improving DRYness.

Example:
```ruby
require 'spec_helper'

describe 'admin/things/index' do
  let(:list_page) { AdminThingsListPage.new }
  let(:thing) { build(:thing, some_attribute: 'some attribute') }

  it 'contains the things we expect' do
    assign(:things, [thing])

    render template: 'admin/things/index'

    list_page.load(rendered)

    expect(list_page.rows.first.some_attribute).to have_text('some attribute')
  end
end
```