Fix issue where front-end is HTTPS, back-end is HTTP, and server issues a redirect.

This handles the case where a back-end web application such as Django or
Rails issues a redirect and automatically decorates the URL with the
protocol with which it was addressed.  If the back-ends are internal and
HTTP-only, then they'll issue a URL with 'http://' as the protocol.
This must be fixed before leaving the proxy.

This also handles the (unusual) case where a back-end speaks only
https://, but the user is deploying node-http-proxy to make that service
available to non-SSL capable browsers.  Works only with 301 and 302
codes.

lib/node-http-proxy/http-proxy.js

@@ -232,6 +232,15 @@ HttpProxy.prototype.proxyRequest = function (req, res, buffer) {
       delete response.headers['transfer-encoding'];
     }
 
+    if ((response.statusCode === 301) || (response.statusCode === 302)) {
+      if (self.source.https && !self.target.https) {
+        response.headers.location = response.headers.location.replace(/^http\:/, 'https:');
+      }
+      if (self.target.https && !self.source.https) {
+        response.headers.location = response.headers.location.replace(/^https\:/, 'http:');
+      }
+    }
+
     // Set the headers of the client response
     res.writeHead(response.statusCode, response.headers);