presto-main/src/main/java/io/prestosql/server/HttpRequestSessionContext.java/HttpRequestSessionContext/splitSessionHeader
class HttpRequestSessionContext:    private static List<String> splitSessionHeader(Enumeration<String> headers)
    {
        Splitter splitter = Splitter.on(',').trimResults().omitEmptyStrings();
        return Collections.list(headers).stream()
                .map(splitter::splitToList)
                .flatMap(Collection::stream)
                .collect(toImmutableList());
    }