fix #2280: when parsing YAML chunk option, label should be an unamed …

…value

thanks @gadenbuie

R/parser.R

@@ -310,7 +310,7 @@ partition_chunk = function(engine, code) {
   }
 
   # normalize field name 'id' to 'label' if provided
-  meta$label = unlist(meta[c('label', 'id')])[1]
+  meta$label = unlist(meta[c('label', 'id')])[[1]]
   meta$id = NULL
   # convert any option with fig- into fig. and out- to out.
   names(meta) = sub('^(fig|out)-', '\\1.', names(meta))