Use "long long" instead of "NSInteger" on byte count.

Byte count should always be 64bit integer. Otherwise, AFNetworking won't work on streams
larger than 4GB in 32bits mode. This commit revert the PR #1772 and e94a52c.
Merge pull request #1892 from yllan/reverse64bit

Use "long long" instead of "NSInteger" on byte count.