Use correct names for arguments fields

Extensions tend to go in the arguments table, which is serialised
as-is rather than going through name translation. Most of these
(except 'alternate-exchange') have an 'x-' prefix.

I'd also forgotten to include 'x-message-ttl', and I was mutating the
passed-in arguments object.

lib/channel_api.js

@@ -82,15 +82,18 @@ C.assertQueue = function(queue, options) {
   queue = queue || '';
   options = options || {};
   var args;
-  if (options.expires ||
+  if (options.messageTtl ||
+      options.expires ||
       options.deadLetterExchange ||
       options.deadLetterRoutingKey ||
       options.maxLength) {
-    args = options.arguments || {};
-    args.expires = options.expires;
-    args.deadLetterExchange = options.deadLetterExchange;
-    args.deadLetterRoutingKey = options.deadLetterRoutingKey;
-    args.maxLength = options.maxLength;
+    args = (options.arguments) ?
+      Object.create(options.arguments) : {};
+    args['x-expires'] = options.expires;
+    args['x-message-ttl'] = options.messageTtl;
+    args['x-dead-letter-exchange'] = options.deadLetterExchange;
+    args['x-dead-letter-routing-key'] = options.deadLetterRoutingKey;
+    args['x-max-length'] = options.maxLength;
   }
 
   var fields = {
@@ -165,8 +168,10 @@ C.assertExchange = function(exchange, type, options) {
   options = options || {};
   var args;
   if (options.alternateExchange) {
-    args = options.arguments || {};
-    args.alternateExchange = options.alternateExchange;
+    args = (options.arguments) ?
+      Object.create(options.arguments) : {};
+    // NB no 'x-' prefix for some reason
+    args['alternate-exchange'] = options.alternateExchange;
   }
   var fields = {
     exchange: exchange,