Keep character case of response header keys

http-party#1029

Author: pachirel

lib/http-proxy/passes/web-outgoing.js

@@ -87,15 +87,16 @@ module.exports = { // <--
     if (typeof rewriteCookieDomainConfig === 'string') { //also test for ''
       rewriteCookieDomainConfig = { '*': rewriteCookieDomainConfig };
     }
-    Object.keys(proxyRes.headers).forEach(function(key) {
-      var header = proxyRes.headers[key];
+    for (var i = 0; i < proxyRes.rawHeaders.length; i = i + 2) {
+      var key = proxyRes.rawHeaders[i];
+      var header = proxyRes.rawHeaders[i + 1];
       if (header != undefined) {
         if (rewriteCookieDomainConfig && key.toLowerCase() === 'set-cookie') {
           header = common.rewriteCookieDomain(header, rewriteCookieDomainConfig);
         }
         res.setHeader(String(key).trim(), header);
       }
-    });
+    };
   },
 
   /**