public boolean setIfAbsent(HeaderName headerName, String value) {
        Objects.requireNonNull(value, "value");
        String normalName = Objects.requireNonNull(headerName, "headerName").getNormalised();
        return setIfAbsentNormal(headerName.getName(), normalName, value);
    }
Adds the name and value to the headers, except if the name is already present.  Unlike
{@link #set(HeaderName, String)}, this method does not accept a {@code null} value.

@return if the value was successfully added.
zuul-core/src/main/java/com/netflix/zuul/message/Headers.java