fix(escapeSpecialCharsWithinTagAttributes): add ~ and = to escaped chars

dist/showdown.js

@@ -1682,7 +1682,7 @@ showdown.subParser('escapeSpecialCharsWithinTagAttributes', function (text) {
     return wholeMatch
       .replace(/(.)<\/?code>(?=.)/g, '$1`')
     //tag = showdown.helper.escapeCharacters(tag, '\\`*_', false);
-      .replace(/([\\`*_])/g, showdown.helper.escapeCharactersCallback);
+      .replace(/([\\`*_ ~=])/g, showdown.helper.escapeCharactersCallback);
   });
 
   return text;

src/subParsers/escapeSpecialCharsWithinTagAttributes.js

@@ -13,7 +13,7 @@ showdown.subParser('escapeSpecialCharsWithinTagAttributes', function (text) {
     return wholeMatch
       .replace(/(.)<\/?code>(?=.)/g, '$1`')
     //tag = showdown.helper.escapeCharacters(tag, '\\`*_', false);
-      .replace(/([\\`*_])/g, showdown.helper.escapeCharactersCallback);
+      .replace(/([\\`*_ ~=])/g, showdown.helper.escapeCharactersCallback);
   });
 
   return text;