Reset AFEventSource#offset when opening a connection

Prevents exception at https://github.com/AFNetworking/AFRocketClient/blob/49b6fde5a9688f7c4014b3083ba86435c3145767/AFRocketClient/AFEventSource.m#L288

Author: clayallsopp

AFRocketClient/AFEventSource.m

@@ -203,6 +203,7 @@ - (BOOL)open:(NSError * __autoreleasing *)error {
 
     [self.lock lock];
     self.state = AFEventSourceConnecting;
+    self.offset = 0;
 
     self.requestOperation = [[AFHTTPRequestOperation alloc] initWithRequest:self.request];
     self.requestOperation.responseSerializer = [AFServerSentEventResponseSerializer serializer];