`assert_dom_attributes_equal` helper

In Activeform we test a lot of inputs:

```ruby
node = css_select("input#user_name").first
assert_equal "user[name]", node.attr("name")
assert_equal "text", node.attr("type")
assert_equal "input-big", node.attr("class")
```

This could be replaced with one helper:

```ruby
node = css_select("input#user_name").first
assert_dom_attributes_equal({name: "user[name", type: "text", class: "input-big"})
```