Make link to first chapter active in index page

Makes both pages more consistent, and also the previous test pass

src/renderer/html_handlebars/hbs_renderer.rs

@@ -116,7 +116,7 @@ impl HtmlHandlebars {
         if ctx.is_index {
             ctx.data.insert("path".to_owned(), json!("index.md"));
             ctx.data.insert("path_to_root".to_owned(), json!(""));
-            ctx.data.insert("is_index".to_owned(), json!("true"));
+            ctx.data.insert("is_index".to_owned(), json!(true));
             let rendered_index = ctx.handlebars.render("index", &ctx.data)?;
             let rendered_index =
                 self.post_process(rendered_index, &ctx.html_config.playground, ctx.edition);

src/renderer/html_handlebars/helpers/toc.rs

@@ -57,6 +57,11 @@ impl HelperDef for RenderToc {
         out.write("<ol class=\"chapter\">")?;
 
         let mut current_level = 1;
+        // The "index" page, which has this attribute set, is supposed to alias the first chapter in
+        // the book, i.e. the first link. There seems to be no easy way to determine which chapter
+        // the "index" is aliasing from within the renderer, so this is used instead to force the
+        // first link to be active. See further below.
+        let mut is_first_chapter = ctx.data().get("is_index").is_some();
 
         for item in chapters {
             // Spacer
@@ -126,7 +131,8 @@ impl HelperDef for RenderToc {
                 out.write(&tmp)?;
                 out.write("\"")?;
 
-                if path == &current_path {
+                if path == &current_path || is_first_chapter {
+                    is_first_chapter = false;
                     out.write(" class=\"active\"")?;
                 }