fixed unicode range testing one Firefox

lib/single-file/css-fonts-minifier.js

@@ -246,7 +246,7 @@ this.fontsMinifier = this.fontsMinifier || (() => {
 
 	function testUnicodeRange(doc, rule) {
 		const unicodeRange = rule.style.getPropertyValue("unicode-range");
-		const docContent = doc.body.outerText;
+		const docContent = doc.body.innerText;
 		if (unicodeRange) {
 			const unicodeRanges = unicodeRange.split(REGEXP_COMMA);
 			const result = unicodeRanges.filter(rangeValue => {