Inconsistent caching of search facets on results page

[mattieburkert]

We need to make sure the filters on the Search Results page are consistently maintaining existing parameters and only ever narrowing, rather than expanding, the user's search.

To reproduce bug: Advanced Search on role="Doctor S" and theatre="Lincoln's Inn (All)". Returns 66 results. On results page, use filters on lefthand side of page to narrow results to Date=after 1728. Desired behavior would be to return 22 results, as would happen by searching all three parameters at once (role, theatre, and date). Instead, theatre parameter is dropped from the search, and instead, 1977 results are returned, reflecting only the combination of the role and date range.