[Quick fix] iOS Release network requests not completing

[teodorciuraru]

Dependencies

"react": "16.13.1",
"react-native": "0.63.4",
"rn-fetch-blob": "^0.12.0"

Problem

Network task fails to complete in Release mode (iOS) and seem to be blocking here, where self.task becomes nil and [nil resume] is called.

...
self.task = [session dataTaskWithRequest:req];
[self.task resume]; // self.task = nil

Workaround

Avoiding self.task makes the network requests work again, so we suspect a problem around it.

// self.task = [session dataTaskWithRequest:req];
// [self.task resume];
[[session dataTaskWithRequest:req] resume];

Proposal Kindly select an option that works for both of us so a PR can be submitted:

1. Avoid using the self.task property altogether. (probably not possible because it's used on cancelRequest)
2. Should the task be declared weak? Declare self.task as strong.
3. Postponing assignment:

   NSURLSessionDataTask *task = [session dataTaskWithRequest:req];
   [task resume];
   self.task = task;

Work will be started immediately after response, because this issue is critical to us.

Edit: Submitted option 3 as a PR, cause it looks the least intrusive and would really help.