Add test for hard line break in heading

commonmark/src/test/java/org/commonmark/renderer/markdown/MarkdownRendererTest.java

@@ -29,6 +29,7 @@ public class MarkdownRendererTest {
         assertRoundTrip("##### foo\n");
         assertRoundTrip("###### foo\n");
 
+        assertRoundTrip("Foo\nbar\n===\n");
         assertRoundTrip("Foo  \nbar\n===\n");
         assertRoundTrip("[foo\nbar](/url)\n===\n");