ftw.solr compatibility hacks

1) solr modifies the query inplace, so we copy it
  https://github.com/4teamwork/ftw.solr/issues/41

2) solr does not support 'path' in the query properly, so we remove it if it is '/' anyways:
  https://github.com/4teamwork/ftw.solr/issues/42

@maethu