Reset `AFEventSource#offset` when opening a connection

Prevents exception at https://github.com/AFNetworking/AFRocketClient/blob/49b6fde5a9688f7c4014b3083ba86435c3145767/AFRocketClient/AFEventSource.m#L288
Merge branch 'reset_offset'
Merge branch 'master' of github.com:usepropeller/AFRocketClient