Handle unspecified or unknown languages when editing code blocks

Use markdown-edit-code-block-default-mode for indirect editing when no
language name is given or when a suitable mode cannot be determined.

Closes GH-284

CHANGES.md

@@ -65,6 +65,8 @@
         features.  ([GH-222][])
     -   Fix font-lock for inline code inside italics and bold.
         ([GH-275][])
+    -   Make code block language detection handle unspecified
+        or unknown code block languages.  ([GH-284][])
 
   [gh-171]: https://github.com/jrblevin/markdown-mode/issues/171
   [gh-216]: https://github.com/jrblevin/markdown-mode/issues/216
@@ -97,6 +99,7 @@
   [gh-275]: https://github.com/jrblevin/markdown-mode/issues/275
   [gh-276]: https://github.com/jrblevin/markdown-mode/issues/276
   [gh-277]: https://github.com/jrblevin/markdown-mode/pull/277
+  [gh-284]: https://github.com/jrblevin/markdown-mode/issues/284
 
 # Markdown Mode 2.3
 

markdown-mode.el

@@ -9182,9 +9182,8 @@ position."
                (end (and bounds (goto-char (nth 1 bounds)) (point-at-bol 1))))
           (if (and begin end)
               (let* ((lang (markdown-code-block-lang))
-                     (mode (if lang
-                               (markdown-get-lang-mode lang)
-                             markdown-edit-code-block-default-mode))
+                     (mode (or (and lang (markdown-get-lang-mode lang))
+                               markdown-edit-code-block-default-mode))
                      (edit-indirect-guess-mode-function
                       (lambda (_parent-buffer _beg _end)
                         (funcall mode))))