Make XmlDeclaration a ProcessingInstruction

This allows Oga to parse documents that contain an XML declaration at a
place other than at the document root. Oga still only assigns the XML
declaration to the document whenever it is at the top-level. This
matches libxml/XML specification behaviour as far as I can tell.

lib/oga.rb

@@ -35,8 +35,8 @@ require 'oga/xml/character_node'
 require 'oga/xml/text'
 require 'oga/xml/comment'
 require 'oga/xml/cdata'
-require 'oga/xml/xml_declaration'
 require 'oga/xml/processing_instruction'
+require 'oga/xml/xml_declaration'
 require 'oga/xml/doctype'
 require 'oga/xml/namespace'
 require 'oga/xml/default_namespace'

lib/oga/xml/generator.rb

@@ -48,12 +48,14 @@ module Oga
             callback = :on_comment
           when Oga::XML::Attribute
             callback = :on_attribute
+          when Oga::XML::XmlDeclaration
+            # This must come before ProcessingInstruction since XmlDeclaration
+            # extends ProcessingInstruction.
+            callback = :on_xml_declaration
           when Oga::XML::ProcessingInstruction
             callback = :on_processing_instruction
           when Oga::XML::Doctype
             callback = :on_doctype
-          when Oga::XML::XmlDeclaration
-            callback = :on_xml_declaration
           when Oga::XML::Document
             callback = :on_document
             children = true

lib/oga/xml/xml_declaration.rb

@@ -1,9 +1,7 @@
 module Oga
   module XML
     # Class containing information about an XML declaration tag.
-    class XmlDeclaration
-      include ToXML
-
+    class XmlDeclaration < ProcessingInstruction
       # @return [String]
       attr_accessor :version
 
@@ -20,9 +18,12 @@ module Oga
       # @option options [String] :encoding
       # @option options [String] :standalone
       def initialize(options = {})
+        super
+
         @version    = options[:version] || '1.0'
         @encoding   = options[:encoding] || 'UTF-8'
         @standalone = options[:standalone]
+        @name       = 'xml'
       end
 
       # @return [String]