FIX Use last of duplicate query params in join_links

When using Controller::join_links to join two links with identical query
params, both query params would be used in the result, ending up with
links that look like `../edit/show/14?locale=en_NZ&locale=mi_NZ`

This patch eliminates duplicate query params, so only the last one
for any key is present in the output.
Merge pull request #2435 from hafriedlander/fix/join_links_query_params

FIX Use last of duplicate query params in join_links