Ignore default NS when serializing elements.

When a default namespace is set (using xmlns="...") the method
XML::Element#to_xml should _not_ include the namespace prefix in the output.

lib/oga/xml/element.rb

@@ -211,7 +211,7 @@ module Oga
       # @return [String]
       #
       def to_xml
-        ns    = namespace ? "#{namespace}:" : ''
+        ns    = namespace_name ? "#{namespace_name}:" : ''
         body  = children.map(&:to_xml).join('')
         attrs = ''
 

spec/oga/xml/element_spec.rb

@@ -315,6 +315,16 @@ describe Oga::XML::Element do
 
       instance.to_xml.should == '<p><!--foo--></p>'
     end
+
+    example 'generate the corresponding XML when using a default namespace' do
+      namespace = Oga::XML::Namespace.new(:name => 'xmlns', :uri => 'foo')
+      instance  = described_class.new(
+        :name       => 'foo',
+        :namespaces => {'xmlns' => namespace}
+      )
+
+      instance.to_xml.should == '<foo></foo>'
+    end
   end
 
   context '#inspect' do