Fix serialization of ATTLIST is incorrect (#234)

GitHub: fix #233

Changed so that "<!ATTLIST" is added to AttDecl contents - simliar to
how it already was with "<!ENTITY"
Updated a test case to test for this

lib/rexml/parsers/baseparser.rb

@@ -378,7 +378,7 @@ def pull_event
               md = @source.match(Private::ATTLISTDECL_END, true)
               raise REXML::ParseException.new( "Bad ATTLIST declaration!", @source ) if md.nil?
               element = md[1]
-              contents = md[0]
+              contents = "<!ATTLIST" + md[0]
 
               pairs = {}
               values = md[0].strip.scan( ATTDEF_RE )

test/test_core.rb

@@ -882,6 +882,7 @@ def test_attlist_decl
       ]>
       <a xmlns:three='xxx' three='yyy'><one:b/><three:c/></a>
       EOL
+      assert_equal '<!ATTLIST blah xmlns CDATA "foo">', doc.doctype.children[0].to_s.gsub(/\s+/, " ")
       assert_equal 'gobble', doc.root.attributes['bar']
       assert_equal 'xxx', doc.root.elements[2].namespace
       assert_equal 'two', doc.root.elements[1].namespace