convert font family name to lowercase

lib/single-file/rules-minifier.js

@@ -138,7 +138,7 @@ this.rulesMinifier = this.rulesMinifier || (() => {
 	}
 
 	function getFontFamily(fontFamily) {
-		fontFamily = fontFamily.trim()
+		fontFamily = fontFamily.toLowerCase().trim();
 		if (fontFamily.match(/^'(.*)'$/)) {
 			return fontFamily.replace(/^'(.*)'$/, "$1");
 		} else {