def to_document(input, context = {}, result = nil)
      result = call(input, context, result)
      HTML::Pipeline.parse(result[:output])
    end
def to_document(input, context = {}, result = nil)
      result = call(input, context, result)
      HTML::Pipeline.parse(result[:output])
    end
Like call but guarantee the value returned is a DocumentFragment.
 Pipelines may return a DocumentFragment or a String. Callers that need a
 DocumentFragment should use this method.