Always deepcopy proposal sorts

f48c86adba6023f3f32ca04eff2ce6ff5a61c1c3 introduced using deepcopy of
proposal sorts in cases where manipulation of proposal sorts is
expected, in order to not mess with the module level PROPOSAL_SORTS.

However deepcopy failed if a request which didn't deepcopy has done
previously (e.g. by a user without defined default sorting), as some
code within SolrPager or thereafter does mess with the passed sorts
object.

Therefore we now always deepcopy in order to avoid the above situation.