code: private function adjustHeaders($requestType)
      {
          if (! array_key_exists('Accept', $this->headers) && $requestType != 'HEAD') {
              $this->setHeader('Accept', 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8');
          }
          
          if (! array_key_exists('Accept-Language', $this->headers) && $requestType != 'HEAD') {
              $this->setHeader('Accept-Language', 'en-US;q=0.7,en;q=0.3');
          }
          
          if (! array_key_exists('User-Agent', $this->headers) && $requestType != 'HEAD') {
              $this->setHeader('User-Agent', 'phpGenerics 1.0');
          }
          
          if (! array_key_exists('Connection', $this->headers) || strlen($this->headers['Connection']) == 0) {
              $this->adjustConnectionHeader($requestType);
          }
          
          if (! array_key_exists('Accept-Encoding', $this->headers)) {
              if (function_exists('gzinflate')) {
                  $encoding = 'gzip, deflate';
              } else {
                  $encoding = 'identity';
              }
              $this->setHeader('Accept-Encoding', $encoding);
          }
      }
docs: Adjust the headers by injecting default values for missing keys.