Add deprecation warning to "Referrer" special-casing

re: #3951

Test plan: `npm test`

Author: rileyjshaw

lib/request.js

@@ -14,6 +14,7 @@
  */
 
 var accepts = require('accepts');
+var deprecate = require('depd')('express');
 var isIP = require('net').isIP;
 var typeis = require('type-is');
 var http = require('http');
@@ -73,8 +74,9 @@ req.header = function header(name) {
   var lc = name.toLowerCase();
 
   switch (lc) {
-    case 'referer':
     case 'referrer':
+      deprecate('The "Referrer" header is non-standard, and special-casing for it will be dropped in future versions.');
+    case 'referer':
       return this.headers.referrer
         || this.headers.referer;
     default: