solr_helper.solr_param_quote now allows digits in un-escaped arg

This is for  https://github.com/projectblacklight/blacklight/issues/603;  fix so local param of $pf2_do_not_quote is passed to solr as   :q=>"{!pf2=$pf2_do_not_quote}foo"  rather than having quotes added like so: :q=>"{!pf2='$pf2_do_not_quote'}foo"  which doesn't work.