Added constructed response object in failureblock

Added the constructed response object to the failure response.

Sometimes we get an error and a body we need do something with. now we
can do both.
Have a look at responseObject and this check ```!self.error```

```
- (id)responseObject {
    [self.lock lock];
    if (!_responseObject && [self isFinished] && !self.error) {
        NSError *error = nil;
        self.responseObject = [self.responseSerializer responseObjectForResponse:self.response data:self.responseData error:&error];
        if (error) {
            self.responseSerializationError = error;
        }
    }
    [self.lock unlock];

    return _responseObject;
}
```