Honor the header 'persistent' when it is '1' (in addition to 'true')

This also prevents the warning:
    Use of uninitialized value in string eq at /usr/share/perl5/site_perl/POE/Component/MessageQueue/Message.pm line 119.
Merge pull request #2 from ironcamel/undef-string-eq

Honor the header 'persistent' when it is '1' (in addition to 'true')