HTTP -- get HTTP headers

• Called with the name of an HTTP header field, returns its value. Capitalization and the use of hyphens versus underscores are not significant.
• Syntax: %HTTP%
• Syntax: %HTTP{"Header-name"}%
• Examples:

  %HTTP%
  %HTTP{"Accept-language"}%	en-US,en;q=0.5
  %HTTP{"User-Agent"}%	CCBot/2.0 (https://commoncrawl.org/faq/)

• Note: You can see the HTTP headers your browser sends to the server on a number of sites e.g. http://www.ericgiguere.com/tools/http-header-viewer.html
• Related: HTTPS, REMOTE_ADDR, REMOTE_PORT, REMOTE_USER