the official spec says that multiple values per key should  only  be allowed if the key  also  is specced to support comma separated values in a single key  i e  if  foo x y  is a valid header  then  foo x foo y  should be as well  in all other cases  supporting multiple header values is not in spec the problem is that it is   dumb   for all the server implementation code to have to check for whether or not a header value is a list  and it is also   dumb   to make single value headers lists just for the sake of consistency  so  happy to change this if you want me to  but i do not like the tradeoffs it leads to 