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.

spec/oga/xml/element_spec.rb

@@ -464,6 +464,13 @@ describe Oga::XML::Element do
 
       element.to_xml.should == '<link />'
     end
+
+    it 'generates the XML for an empty explicitly closed HTML element' do
+      element = described_class.new(:name => 'html')
+      document = Oga::XML::Document.new(:type => :html, :children => [element])
+
+      element.to_xml.should == '<html></html>'
+    end
   end
 
   describe '#inspect' do

spec/oga/xml/generator_spec.rb

@@ -29,6 +29,27 @@ describe Oga::XML::Generator do
       end
     end
 
+    describe 'using an HTML Document as the root node' do
+      it 'returns a String' do
+        element = Oga::XML::Element.new(name: 'foo')
+        doc = Oga::XML::Document.new(children: [element], type: :html)
+        output = described_class.new(doc).to_xml
+
+        output.should == '<foo></foo>'
+      end
+    end
+
+    describe 'using an HTML Document as the root node with nested elements' do
+      it 'returns a String' do
+        el2 = Oga::XML::Element.new(name: 'bar')
+        el1 = Oga::XML::Element.new(name: 'foo', children: [el2])
+        doc = Oga::XML::Document.new(children: [el1], type: :html)
+        output = described_class.new(doc).to_xml
+
+        output.should == '<foo><bar></bar></foo>'
+      end
+    end
+
     describe 'using Element nodes with siblings' do
       it 'returns a String' do
         root = Oga::XML::Element.new(
@@ -78,5 +99,32 @@ describe Oga::XML::Generator do
         described_class.new(element2).to_xml.should == '<b />'
       end
     end
+
+    describe 'using a parsed HTML document' do
+      it 'returns a String with the same formatting as the input document' do
+        input = <<-EOF
+<!DOCTYPE html>
+<html>
+  <head>
+    <title>Hello</title>
+    <meta charset="utf-8" />
+  </head>
+  <body>
+    <p>Hello</p>
+    <ul>
+      <li>Hello</li>
+      <li></li>
+      Hello
+    </ul>
+  <div></div></body>
+</html>
+        EOF
+
+        doc = Oga.parse_html(input)
+        output = described_class.new(doc)
+
+        output.to_xml.should == input
+      end
+    end
   end
 end