/consultations retains query params when redirecting

The consultations controller index action redirects onto the publications
controller setting publication_filter_option=consultations. There are
links which use the consultations_path helper adding in pre-set filter
values. These were getting dropped by the redirect.

For an example, see the "See all consultations" link on a topic page
(https://www.gov.uk/government/topics/public-health). The link takes
the user to the publications index showing all consultations instead
of only consultations related to that topic.
Merge pull request #1392 from alphagov/fix-consultations-index-redirect

/consultations retains query params when redirecting