convert media to lower case

lib/single-file/single-file-core.js

@@ -784,7 +784,7 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 				.map(async element => {
 					let mediaText;
 					if (element.media) {
-						mediaText = element.media;
+						mediaText = element.media.toLowerCase();
 					}
 					this.stylesheets.set(element, { mediaText });
 					const options = { maxResourceSize: this.options.maxResourceSize, maxResourceSizeEnabled: this.options.maxResourceSizeEnabled, url: this.options.url, charset: this.charset, compressCSS: this.options.compressCSS };