Fix issue with missing namespaces

Newer versions of Nokogiri (>= 1.12.0) can use libxml >= 2.9.12 which breaks the
automatic addition of the parent namespace when adding a child.
This was already the case for JRuby, so added the same workaround
for when nokogiri is using this newer version of libxml.

Ref: sparklemotion/nokogiri#425

.rspec

@@ -0,0 +1 @@
+--format progress --color

Rakefile

@@ -16,7 +16,7 @@ RDoc::Task.new(:rdoc) do |rdoc|
   rdoc.rdoc_dir = 'rdoc'
   rdoc.title    = 'Saml'
   rdoc.options << '--line-numbers'
-  rdoc.rdoc_files.include('README.rdoc')
+  rdoc.rdoc_files.include('README.md')
   rdoc.rdoc_files.include('lib/**/*.rb')
 end
 

lib/xmlmapper.rb

@@ -687,14 +687,16 @@ def to_xml(builder = nil, default_namespace = nil, namespace_override = nil,
           # an empty element will be written to the xml
           #
           if value.nil? && element.options[:single] && element.options[:state_when_nil]
-            # NOTE In JRuby 9.0.4.0+ and Nokogiri version 1.6.8, Nokogiri::XML::Builder::NodeBuilder
-            # does not retain the XML namespace prefix for an element when adding an element
-            # to a parent node.
+            #
+            # NOTE
+            # In JRuby 9.0.4.0+ and Nokogiri version 1.6.8 or with Nokogiri version >= 1.12.0 (libxml >= 2.9.12),
+            # the Nokogiri::XML::Builder::NodeBuilder does not retain the XML namespace prefix for an element
+            # when adding an element to a parent node.
             #
             # The namespace prefix must be specified when adding the node to its parent.
             # This issue manifests when setting an element's :state_when_nil' option to true.
             #
-            # This JRuby workaround is intended for XML element prefixes that originate from a
+            # This workaround is intended for XML element prefixes that originate from a
             # single namespace defined in 'registered_namespaces'. If there are
             # multiple namespaces defined in the 'registered_namespaces' array,
             # then the first namespace is selected.
@@ -707,7 +709,7 @@ def to_xml(builder = nil, default_namespace = nil, namespace_override = nil,
             # 3. Adding namespace-less node to namespaced parent attaches the parent namespace to the child
             # https://github.com/sparklemotion/nokogiri/issues/425
             #
-            if RUBY_ENGINE == 'jruby' && !registered_namespaces.empty?
+            if (RUBY_ENGINE == 'jruby' || Nokogiri.uses_libxml?('>= 2.9.12')) && !registered_namespaces.empty?
               ns = registered_namespaces.keys.first.to_sym
               xml[ns].send("#{tag}_","")
             else

lib/xmlmapper/version.rb

@@ -1,3 +1,3 @@
 module XmlMapper
-  VERSION = "0.7.3"
+  VERSION = '0.8.0'.freeze
 end