Avoid redirect when selecting a filter

[YolCruz]

Is your feature request related to a problem? Please describe.
When a new filter is selected, the URL to be used is missing a tailing slash, causing a double request

Describe the solution you'd like
Add the missing "/" at the end of the URL

Describe alternatives you've considered
Forking the project and open a PR

Additional context
When selecting a filter, 2 GET requests are made

"GET /admin/advfilters/field_choices/textpicks.textpickseen/ip_address HTTP/1.0" 301 0
"GET /admin/advfilters/field_choices/textpicks.textpickseen/ip_address/ HTTP/1.0" 200 15

The first one causes a redirect to the actual URL that needs to be used