task cancelled issue due to improper type casting

[GaborWnuk]

Hello!

First of all - thank you for this excellent library and an amazing job!

I've faced slight problem with 0.10.9 branch - when i've tried to fetch multiple assets on my list view, all requests failed due to task cancelled error even though there was no such event.

I've checked Objective-C implementation and found out there is an issue with type casting in RNFetchBlobNetwork.m:

BOOL isCancelled = [taskSession valueForKey:@"isCancelled"];

NSLog(@"%@", taskSession);
NSLog(@"%d", isCancelled);
if(isCancelled) {
    errMsg = @"task cancelled";
}

NSLog(@"%@", errMsg);

which resulted in:

2017-09-08 14:49:40.266 Pudelek[57084:957459] {
    isCancelled = 0;
    session = "<__NSCFLocalDataTask: 0x7ffd313161c0>{ taskIdentifier: 1 } { completed }";
}
2017-09-08 14:49:40.266 Pudelek[57084:957459] 1
2017-09-08 14:49:40.267 Pudelek[57084:957459] "task cancelled"

This simple pull requests fixed the problem for me.