code: public function setSubject($subject = '', $clear = false)
      {
          if (true===$clear) {
              $this->clear('subject');
          }
          $this->subject = $subject;
          return $this;
      }
docs: Set mail object
  
  @param string $subject The subject content
  @param bool $clear Clear a set content first ? (default is to append a content)
  @return self