Remove blank params from sort_link, close issue 158

Fixes issue 158 (github.com/ernie/ransack/issues/158)

lib/ransack/search.rb

@@ -18,6 +18,8 @@ module Ransack
       @context = Context.for(object, options)
       @context.auth_object = options[:auth_object]
       @base = Nodes::Grouping.new(@context, 'and')
+      params = (params.delete_if { |k, v| v.blank? && v != false }
+        ) if params.present?
       build(params.with_indifferent_access)
     end