assert_has/refute_has by 'label' incorrect if no 'value' given

[ftes]

Given

<form action="/">
  <label>
    Hobbit <input type="text" name="user" value="Frodo" />
  </label>
</form

I expect

|> refute_has("input", label: "Unknown")

to succeed, but it fails with

     Expected not to find any elements with selector "input" with label "Unknown"

     But found 1:

     <input type="text" name="user" value="Frodo"/>

Failing tests see #286