Generating closing element & Doctype XML

This commit fixes two problems:

1. Doctypes introducing too many newlines
2. Elements with siblings and a common parent not being closed properly

== Doctypes

When generating the XML for a doctype the XML::Generator class would
append a trailing newline. This however meant that if the next text node
was also a newline you'd now have two newlines. In previous versions of
Oga this worked because the old XML generation code would call
String#strip on the XML to add after the doctype.

To support this in the new version we perform a lookahead in
XML::Generator#on_doctype to remove any trailing newlines added by this
method in case the first child node is a newline text node.

== Closing Elements

When an element has a sibling following it _and_ does not have any child
nodes it would not be closed properly when generating XML. This is due
to the "until next_node = ..." expression evaluating to true, thus never
executing its body.

There's probably some way to work around this by using the "loop"
method, but considering it's 02:09 I think the current approach is good
enough. Future me will probably hate me for it.

lib/oga/xml/generator.rb

@@ -17,8 +17,8 @@ module Oga
       def initialize(root)
         @start = root
 
-        if @start.respond_to?(:root_node)
-          @html_mode = @start.root_node.html?
+        if @start.respond_to?(:html?)
+          @html_mode = @start.html?
         else
           @html_mode = false
         end
@@ -75,6 +75,10 @@ module Oga
 
             break
           else
+            # Make sure to always close the current element before
+            # moving to any siblings.
+            after_element(current, output) if current.is_a?(Element)
+
             until next_node = current.is_a?(Node) && current.next
               if current.is_a?(Node) && current != @start
                 current = current.parent
@@ -177,6 +181,14 @@ module Oga
           on_doctype(doc.doctype, output)
           output << "\n"
         end
+
+        first_child = doc.children[0]
+
+        # Prevent excessive newlines in case the next node is a newline text
+        # node.
+        if first_child.is_a?(Text) && first_child.text.start_with?("\r\n", "\n")
+          output.chomp!
+        end
       end
 
       # @param [Oga::XML::XmlDeclaration] node