Allow nil for external and system IDs when creating a DTD

The DTD initializer required external and system to be strings,
making it impossible to create an internal DTD without public/system
identifiers. Now nil is accepted for both parameters.

Fixes #215

Author: cfis

ext/libxml/ruby_xml_dtd.c

@@ -183,11 +183,17 @@ static VALUE rxml_dtd_initialize(int argc, VALUE *argv, VALUE self)
           VALUE name, doc, internal;
           rb_scan_args(argc, argv, "23", &external, &system, &name, &doc, &internal);
 
-          Check_Type(external, T_STRING);
-          xpublic = (const xmlChar*) StringValuePtr(external);
+          if (external != Qnil)
+          {
+            Check_Type(external, T_STRING);
+            xpublic = (const xmlChar*) StringValuePtr(external);
+          }
 
-          Check_Type(system, T_STRING);
-          xsystem = (const xmlChar*) StringValuePtr(system);
+          if (system != Qnil)
+          {
+            Check_Type(system, T_STRING);
+            xsystem = (const xmlChar*) StringValuePtr(system);
+          }
 
           if (name != Qnil)
           {

test/test_dtd.rb

@@ -44,6 +44,16 @@ def test_internal_subset
 		assert_equal "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd", xhtml_dtd.system_id
   end
 
+  def test_internal_subset_nil_ids
+    doc = LibXML::XML::Document.new
+    doc.root = LibXML::XML::Node.new('test')
+    dtd = LibXML::XML::Dtd.new(nil, nil, 'test', doc, true)
+    assert_equal('test', dtd.name)
+    assert_nil(dtd.external_id)
+    assert_nil(dtd.uri)
+    assert_equal("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<!DOCTYPE test>\n<test/>\n", doc.to_s)
+  end
+
   def test_external_subset
     xhtml_dtd = LibXML::XML::Dtd.new("-//W3C//DTD XHTML 1.0 Transitional//EN", "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd", nil)
 		assert xhtml_dtd.name.nil?