code: public String getFirstHeaderStringValue(String name) {
      Object value = get(name.toLowerCase(Locale.US));
      if (value == null) {
        return null;
      }
      Class<? extends Object> valueClass = value.getClass();
      if (value instanceof Iterable<?> || valueClass.isArray()) {
        for (Object repeatedValue : Types.iterableOf(value)) {
          return toStringValue(repeatedValue);
        }
      }
      return toStringValue(value);
    }
docs: Returns the first header string value for the given header name.
  
  @param name header name (may be any case)
  @return first header string value or {@code null} if not found
  @since 1.13