fixed Firefox issue when using parseSrcset

lib/single-file/single-file-browser.js

@@ -83,7 +83,7 @@ this.SingleFile = (() => {
 				getComputedStyle,
 				document: doc,
 				serialize: () => getDoctype(doc) + doc.documentElement.outerHTML,
-				parseSrcset,
+				parseSrcset: srcset => parseSrcset(srcset),
 				uglifycss: (content, options) => uglifycss.processString(content, options)
 			};
 		}