Fix description and GFM pipelines conflicting

Consider this command:

    bundle exec rails r "include GitlabMarkdownHelper
    puts markdown('<span>this is a span</span>', pipeline: :description)
    puts markdown('<span>this is a span</span>')"

And the same in the opposite order:

    bundle exec rails r "include GitlabMarkdownHelper
    puts markdown('<span>this is a span</span>')
    puts markdown('<span>this is a span</span>', pipeline: :description)"

Before this change, they would both output:

    <p><span>this is a span</span></p>
    <p>this is a span</p>

That's because `span` is added to the list of whitelisted elements in
the `SanitizationFilter`, but this method tries not to make the same
changes multiple times. Unfortunately,
`HTML::Pipeline::SanitizationFilter::LIMITED`, which is used by the
`DescriptionPipeline`, uses the same Ruby objects for all of its hash
values _except_ `:elements`.

That means that whichever of `DescriptionPipeline` and `GfmPipeline` is
called first would have `span` in its whitelisted elements, and the
second wouldn't.

Fix this by creating an entirely separate hash, before either pipeline
is invoked.

lib/banzai/pipeline/description_pipeline.rb

 module Banzai
   module Pipeline
     class DescriptionPipeline < FullPipeline
+      WHITELIST = Banzai::Filter::SanitizationFilter::LIMITED.deep_dup.merge(
+        elements: Banzai::Filter::SanitizationFilter::LIMITED[:elements] - %w(pre code img ol ul li)
+      )
+
       def self.transform_context(context)
         super(context).merge(
           # SanitizationFilter
-          whitelist: whitelist
+          whitelist: WHITELIST
         )
       end
-
-      private
-
-      def self.whitelist
-        # Descriptions are more heavily sanitized, allowing only a few elements.
-        # See http://git.io/vkuAN
-        whitelist = Banzai::Filter::SanitizationFilter::LIMITED
-        whitelist[:elements] -= %w(pre code img ol ul li)
-
-        whitelist
-      end
     end
   end
 end