Fixed legacy non-sasl auth

I've found a problem in how strophe works with stream:features when searching for non-sasl auth.
Here is an example from http://xmpp.org/extensions/xep-0078.html
```
<stream:features>
    <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
      <mechanism>DIGEST-MD5</mechanism>
      <mechanism>PLAIN</mechanism>
    </mechanisms>
    <auth xmlns='http://jabber.org/features/iq-auth'/>
  </stream:features>
```
As I understand xep-0078 and this example, auth tag should be checked separately and it hasn't to be dependent on mechanisms.