pass a hashref, like we used to, in plugin_from_config
v5.027

        - fix regression in Plugin->plugin_from_config which started passing a
          list of pairs rather than a hashref
additional tests for [NextRelease] Changes munging
use the original Changes content as a base for munging, rather than whatever is in the repo after release

This allows other AfterRelease plugins to copy the Changes file from the built
distribution back to the repo and commit it, before we add in the {{$NEXT}}
line here.