gcloud.storage.iterator.KeyDataIterator chunking is off-by-one

[tseaver]

'get_headers' computes the "Range:" header as '_bytes_written' - '_bytes_written + CHUNK_SIZE':

https://github.com/GoogleCloudPlatform/gcloud-python/blob/1f2aaf3606802f4f02eeb30e22e1082ef7f4594e/gcloud/storage/iterator.py#L215

and the '_bytes_written' gets updated to bump by the size of the chunk the server returns. Consider the initial read, assuming a CHUNK_SIZE of 100:

Range: bytes=0-100

Note that, per RFC2616. this will return up to 101 bytes (both bytes-position values are inclusive):

http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.35.1