Fix for finding nested input elements in SAML HTML form

[aritgithub]

Example of not retrieved input elements wrapped in a div element:

<form action="https://goolex.okta.com/app/goolex_a1_1/exk1hi6xl2jFdGu7A669/sso/saml" method="post">
    <div>
        <input type="hidden" name="SAMLRequest" value="PD9..."/>
        <input type="hidden" name="RelayState" value="RS1"/>
    </div>
    <noscript>
        <div>
            <input type="submit" value="Continue"/>
        </div>
    </noscript>
</form>

[peppelinux]

ciao @aritgithub
is this PR improving the project, is this a bug fix or a special behaviour that's needed only in some cases?

[aritgithub]

Some frameworks generate the SAML html form with the input elements wrapped in other elements, like div, and this misleads the SAMLRequest validation procedure (spid_sp_test.authn_request:SpidSpAuthnReqCheck.test_AuthnRequest).
With this fix the procedure is more generic and should correctly retrieve form input data values in all circumstances.

<form>
  <input .../>
  <input .../>
</form>

and

<form>
  <div>
    <input .../>
    <input .../>
  </div>
</form>

[aritgithub]

The error appears at line 48 of authn_request.py module where try to retrieve a nonexistent reference:
data["SAMLRequest"] = form_dict["SAMLRequest"]

[peppelinux]

Thank you