bug 1031972 - Problem with paging the task list when multiple checkboxes are used for the filter

Replace call to `urlparams`, since it discards repeated url query keys

This alternative just adds/replaces a 'page' key in the existing query string, keeping everything else intact. 
@bobsilverberg Asking about it made me realize that `request.path` isn't even needed. After all, we originally pass an empty string as the url to `urlparams`. All we really want to return is a appropriate query string of the form `?'a=1&b=2`...