fixed srcset concatenation

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

@@ -778,7 +778,7 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 						}
 					}
 				}));
-				resourceElement.setAttribute(attributeName, srcsetValues.join(","));
+				resourceElement.setAttribute(attributeName, srcsetValues.join(", "));
 			}));
 		}