Manually remove stylesheet attr for child docs

Cloning Document.attributes does not preserve a deleted attribute key,
so if the stylesheet attribute does not exist in the original
Document, remove it in all the generated Documents.

Closes #44.

lib/asciidoctor-multipage.rb

@@ -508,6 +508,11 @@ def convert_section(node)
                                         :doctype => doc.doctype,
                                         :header_footer => !doc.attr?(:embedded),
                                         :safe => doc.safe})
+      # Manually remove stylesheet attribute if not set on original document
+      # (why is doc.attributes.clone not adequate?)
+      if ! doc.attributes.has_key?('stylesheet')
+        page.remove_attr('stylesheet')
+      end
       # Retain webfonts attribute (why is doc.attributes.clone not adequate?)
       page.set_attr('webfonts', doc.attr(:webfonts))
       # Save sectnum for use later (a Document object normally has no sectnum)