refactor: handle the uppercase srcset attribute

Author: alexander-akait

src/plugins/attribute-plugin.js

@@ -385,7 +385,7 @@ function processMatch(match, strUntilValue, name, value, index) {
  return;
  }
 
- if (name === 'srcset') {
+ if (name.toLowerCase() === 'srcset') {
  let sourceSet;
 
  try {