use [data copy] to be extra safe

[woshimaliang]

For NSData (the actual concrete type under the hood is OS_dispatch_data), i feel the copy here should be good because if the data is truly immutable, copy will just referenceCount++, and if its NSMutableData, then copy is just right.

[jparise]

What do you think about changing the PINNSURLSessionDataTaskResult.data property to copy instead?

PINFuture/PINFuture/Classes/Categories/PINNSURLSessionDataTaskResult.m

Line 14 in 689ec2e

@property (nonatomic) NSData * _Nullable data;