render_search_to_page_title renders difficult-to-read output if filter value labels contain markup

[barmintor]

Filter value labels from ItemPresenter may contain markup suitable for the constraints display which cannot be rendered in a page <title>.

This makes the content rendered by CatalogHelperBehavior#render_search_to_page_title difficult to read.

See also: https://github.com/projectblacklight/blacklight_range_limit/issues/49