bug #3638 Fix call to deprecated "convertToHtml" method (jderusse)

This PR was submitted for the 3.x branch but it was merged into the 2.x branch instead.

Discussion
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Fix call to deprecated "convertToHtml" method

note: We can not rely on the new interface `ConverterInterface` because this interface already existed in [version 1.0](https://github.com/thephpleague/commonmark/blob/1.0/src/ConverterInterface.php)

Commits
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779fdd0 Fix call to deprecated "convertToHtml" method

extra/markdown-extra/LeagueMarkdown.php

@@ -16,14 +16,20 @@
 class LeagueMarkdown implements MarkdownInterface
 {
     private $converter;
+    private $legacySupport;
 
     public function __construct(CommonMarkConverter $converter = null)
     {
         $this->converter = $converter ?: new CommonMarkConverter();
+        $this->legacySupport = !method_exists($this->converter, 'convert');
     }
 
     public function convert(string $body): string
     {
-        return $this->converter->convertToHtml($body);
+        if ($this->legacySupport) {
+            return $this->converter->convertToHtml($body);
+        }
+
+        return $this->converter->convert($body);
     }
 }