OMEMO: Fix parsing prekey boolean value

Though the stanza in the bug report is OK, looking closely again and again
and comparing with previous stanzas reveals a difference, worth to be checked.
There is a payload but no key in this stanza only a prekey.
Actually this should be also properly handled. Looking further:
   ... <key prekey='1' ...
XEP-0384 says:
   ...="true" ...
but '1' is also allowed by the XML standard.
It turns out, Gajim accepts only 'true' here. Testing with simple
   prekey = kn.getAttr('prekey') == 'true' or kn.getAttr('prekey') == '1'
fixed the issue.

The code in question has appeared since module refactoring deba7180
in February 2019.

Fixes #109