Fixed of losing context of existing headers in response

It fixes losing context in response in case when here 3 or more Set-Cookie headers.
Closes: #1621

Author: skokalin

src/nodejs/unit-http/http_server.js

@@ -153,7 +153,7 @@ ServerResponse.prototype._removeHeader = function _removeHeader(lc_name) {
         this.headers_count -= value.length;
         this.headers_len -= value.length * name_len;
 
-        value.forEach(function(val) {
+        value.forEach(val => {
             this.headers_len -= Buffer.byteLength(val + "", 'latin1');
         });