Add nil check for Ruby 2.3

It's possible for `filters` to be `nil`, which causes Ruby 2.3 to raise
an error: `TypeError: can't dup NilClass`

lib/stripe/api_operations/list.rb

@@ -11,7 +11,7 @@ def list(filters = {}, opts = {})
 
         # set filters so that we can fetch the same limit, expansions, and
         # predicates when accessing the next and previous pages
-        obj.filters = filters.dup
+        obj.filters = filters.dup unless filters.nil?
         obj
       end
     end