Merge pull request marcuswestin#73 from noahlu/master

prevent insert nil into NSDictionary

Author: marcuswestin

WebViewJavascriptBridge/WebViewJavascriptBridge.m

@@ -174,6 +174,10 @@ - (void)_flushMessageQueue {
             NSString* callbackId = message[@"callbackId"];
             if (callbackId) {
                 responseCallback = ^(id responseData) {
+                    if (responseData == nil) {
+                        responseData = @{};
+                    }
+                    
                     WVJBMessage* msg = @{ @"responseId":callbackId, @"responseData":responseData };
                     [self _queueMessage:msg];
                 };