Preserve whitespace display

This transformer turns whitespace in posted content into <br>s, taking into account that some whitespace (such as those between block elements) should not receive that treatment.

def replace_text_with_node(node, text, node_to_insert)
		original_content = node.text
		start = node.text.index(text)
		# the stuff before stays in the current node
		node.content = original_content[0, start]
		# add the new node
		node.add_next_sibling(node_to_insert)
		# the stuff after becomes a new text node
		node_to_insert.add_next_sibling(Nokogiri::XML::Text.new(original_content[start + text.size, original_content.size], node.document))
		return [node, node.next_sibling, node.next_sibling.next_sibling]
end

def has_ancestor(node, ancestor_node_name)
	until node.nil?
		return true if node.name == ancestor_node_name
		node = node.parent
	end
	return false
end

def element_is_block(node)
	return false if node.nil?
	# https://github.com/rgrove/sanitize/issues/108
	d = Nokogiri::HTML::ElementDescription[node.name]
	return !d.nil? && d.block?
end

fix_whitespace = lambda do |env|
	node = env[:node]
	return unless node.text?
	return if has_ancestor(node, 'pre')
	node.content = node.content.lstrip if element_is_block(node.previous_sibling)
	node.content = node.content.rstrip if element_is_block(node.next_sibling)
	return if node.text.empty?
	return unless node.text.include?("\n")
	resulting_nodes = replace_text_with_node(node, "\n", Nokogiri::XML::Node.new('br', node.document))
	# not required in Sanitize 3
	# sanitize the new nodes ourselves; they won't be picked up otherwise.
#	resulting_nodes.delete(node)
#	resulting_nodes.each do |new_node|
#		Sanitize.clean_node!(new_node, env[:config])
#	end
end