FIX: doc.root add omment or processinginstruction sibling

FIX:    doc.root cann't add_next_sibling comment or processinginstruction

    pi   = Nokogiri::XML::ProcessingInstruction.new(doc, 'xml-xxx', 'yy="zz"')
    comm = Nokogiri::XML::Comment.new(doc, 'xxx')

    doc.root.add_previous_sibling(pi)     # ok
    doc.add_child(pi)                     # RuntimeError: Document already has a root node
    doc.root.add_next_sibling(pi)         # ArgumentError: A document may not have multiple root nodes

    doc.root.add_previous_sibling(comm)   # ArgumentError: A document may not have multiple root nodes
    doc.root.add_next_sibling(comm)       # ArgumentError: A document may not have multiple root nodes