up-storage: don't send X-Forwarded-For for proxies

It's not that important a feature to have, and it could break
some proxies.

ref #254

lib/up-storage.js

@@ -361,11 +361,19 @@ Storage.prototype.search = function(startkey, options) {
 
 Storage.prototype._add_proxy_headers = function(req, headers) {
   if (req) {
-    headers['X-Forwarded-For'] = (
-      req && req.headers['x-forwarded-for']
-      ? req.headers['x-forwarded-for'] + ', '
-      : ''
-    ) + req.connection.remoteAddress
+    // Only submit X-Forwarded-For field if we don't have a proxy selected
+    // in the config file.
+    //
+    // Otherwise misconfigured proxy could return 407:
+    // https://github.com/rlidwka/sinopia/issues/254
+    //
+    if (!this.proxy) {
+      headers['X-Forwarded-For'] = (
+        req && req.headers['x-forwarded-for']
+        ? req.headers['x-forwarded-for'] + ', '
+        : ''
+      ) + req.connection.remoteAddress
+    }
   }
 
   // always attach Via header to avoid loops, even if we're not proxying