Use the defaultTimeout value if no TTURLRequest is given.

This fixes a bug where a timeout value of 0.0 seconds is used in a NSURLRequest created without a given TTURLRequest. This causes the NSURLRequest to timeout immediately and fail.
The TTURLRequest is nil when there are several requests in a TTReuestLoader.

src/Three20Network/Sources/TTURLRequestQueue.m

@@ -499,7 +499,7 @@ - (NSURLRequest*)createNSURLRequest:(TTURLRequest*)request URL:(NSURL*)URL {
 
   NSTimeInterval usedTimeout = request.timeoutInterval;
 
-  if (usedTimeout < 0.0) {
+  if (usedTimeout < 0.0 || request == nil) {
     usedTimeout = self.defaultTimeout;
   }