Replace null characters with 0xFFFD instead of strip

lib/parser_block.js

@@ -124,7 +124,7 @@ ParserBlock.prototype.parse = function (src, md, env, outTokens) {
   src = src.replace(NEWLINES_RE, '\n');
 
   // Strin NULL characters
-  src = src.replace(NULL_RE, '');
+  src = src.replace(NULL_RE, '\uFFFD');
 
   // Replace tabs with proper number of spaces (1..4)
   if (src.indexOf('\t') >= 0) {

test/misc.js

@@ -167,7 +167,7 @@ describe('Misc', function () {
   it('Should strip (or replace) NULL characters', function () {
     var md = markdownit();
 
-    assert.strictEqual(md.render('foo\u0000bar'), '<p>foobar</p>\n');
+    assert.strictEqual(md.render('foo\u0000bar'), '<p>foo\uFFFDbar</p>\n');
   });
 
   it('Should correctly parse strings without tailing \\n', function () {