public boolean setIfAbsent(String headerName, String value) {
        Objects.requireNonNull(value, "value");
        String normalName = HeaderName.normalize(Objects.requireNonNull(headerName, "headerName"));
        return setIfAbsentNormal(headerName, normalName, value);
    }
Adds the name and value to the headers, except if the name is already present.  Unlike
{@link #set(String, 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