bugfix: form input discovery

[JohaJung]

BeatifulSoup considers inputs as equal if they have the same HTML representation. This broke checks like tag in list_of_tags in an unexpected way.

In cases like

        <form method="POST" id="outer_inputs_form">
            <input name="bar" type="text" value="bar">
        </form>
        <!-- this input is equal (in the sense beautifulSoup checks equality)
             to that in outer_inputs_form -->
        <input name="bar" type="text" value="bar">

webtest v3.0.5 consideres the outer bar input belonging to outer_inputs_form.

I'm sorry that I overlooked that in #267 .

Now, I added a test which reveals this bug, and fixed the input field discovery (which I adjusted in #267 and thereby introduced that bug).

@gawel not sure how severe that bug is, but it has the potential to break stuff if webtest is used on pages with multiple forms on one page.