/**
  * Constructor.
  *
  * @param string|null $value
  */
 public function __construct($value = null)
 {
     if (null !== $value) {
         GenericHeader::assertHeaderValue($value);
         $this->value = $value;
     }
 }
 /**
  * Set header value.
  *
  * @param string $value
  *
  * @return $this
  */
 public function set($value)
 {
     GenericHeader::assertHeaderValue($value);
     $this->value = $value;
     return $this;
 }