vweb assets: the assets cache files are now immutable after creation. (#1454)

vlib/vweb/assets/assets.v

@@ -77,13 +77,13 @@ fn (am mut AssetManager) combine(asset_type string, to_file bool) string {
 	mut out := ''
 	// use cache 
 	if os.file_exists(out_file) {
+		if to_file {
+			return out_file
+		}    	
 		cached := os.read_file(out_file) or {
 			return ''
 		}
-		out = cached
-		if !to_file {
-			return out
-		}
+		return cached
 	}
 	// rebuild
 	for asset in am.get_assets(asset_type) {