explicitly convert errata keywords to String

This is to fix the following tracebacks:

Caused by: java.lang.ClassCastException:
com.redhat.rhn.domain.errata.impl.PublishedKeyword cannot be cast to
java.lang.String
        at
com.redhat.rhn.common.util.StringUtil.join(StringUtil.java:415)
        at
com.redhat.rhn.frontend.action.errata.EditAction.unspecified(EditAction.java:87)
...

Caused by: java.lang.ClassCastException:
com.redhat.rhn.frontend.dto.ErrataKeyword cannot be cast to
java.lang.String
        at
com.redhat.rhn.common.util.StringUtil.join(StringUtil.java:415)
        at
com.redhat.rhn.frontend.action.errata.ErrataDetailsSetupAction.execute(ErrataDetailsSetupAction.java:65)
Automatic commit of package [spacewalk-java] release [2.2.95-1].