Handle odd numbers of NodeFilters to be configured

The previous method of merging the results from the NodeFilter instances was breaking because Ruby was treating it as a series of 2-element arrays.

If you had 0, 1 or 2 node filters configured, it would work fine; if you added a third, it would break with an `ArgumentError`. Adding a fourth makes it work again.

Fixes #388

lib/html_pipeline.rb

@@ -178,7 +178,7 @@ def call(text, context: {}, result: {})
       end
     end
 
-    result = result.merge(Hash[*@node_filters.collect(&:result).flatten])
+    @node_filters.collect(&:result).reduce(result, :merge)
     @node_filters.each(&:reset!)
 
     result