Fix double setext heading underlines

Related-to: GH-22.

Author: wooorm

Diff for: src/construct/document.rs

@@ -423,7 +423,7 @@ pub fn flow_end(tokenizer: &mut Tokenizer) -> State {
     if !document_lazy_continuation_current && !child.events.is_empty() {
         let before = skip::opt_back(&child.events, child.events.len() - 1, &[Name::LineEnding]);
         let name = &child.events[before].name;
-        if name == &Name::Content {
+        if name == &Name::Content || name == &Name::HeadingSetextUnderline {
             document_lazy_continuation_current = true;
         }
     }

Diff for: src/construct/heading_setext.rs

@@ -92,13 +92,12 @@ pub fn start(tokenizer: &mut Tokenizer) -> State {
         && !tokenizer.pierce
         // Require a paragraph before.
         && (!tokenizer.events.is_empty()
-            && tokenizer.events[skip::opt_back(
+            && matches!(tokenizer.events[skip::opt_back(
                 &tokenizer.events,
                 tokenizer.events.len() - 1,
                 &[Name::LineEnding, Name::SpaceOrTab],
             )]
-            .name
-                == Name::Content)
+            .name, Name::Content | Name::HeadingSetextUnderline))
     {
         tokenizer.enter(Name::HeadingSetextUnderline);
 

Diff for: tests/heading_setext.rs

@@ -249,6 +249,12 @@ fn heading_setext() -> Result<(), String> {
         "should prefer a setext heading over an interrupting list"
     );
 
+    assert_eq!(
+        to_html("[a]: b\n=\n="),
+        "<h1>=</h1>",
+        "should support a two setext heading underlines after a definition, as a setext heading"
+    );
+
     assert_eq!(
         to_html("> ===\na"),
         "<blockquote>\n<p>===\na</p>\n</blockquote>",