fix object mgr _fetch_chunker() to use chunk_size appropriately
fix object mgr _fetch_chunker() to use chunk_size appropriately

- the get request was using the chunk_size incorrectly and therefore
causing data corruption on fetched objects.
  - request was ending up retrieving chunk_size+1 bytes (0 - chunk_size)
  - the next request would start at chunk_size, thus overlapping
    the previous chunk by 1 byte. (0-4096, 4096-8192, etc...)
    [last byte (4096) of previous read was re-read as the first byte
    on next fetch]

sivel/pyrax,ddaeschler/pyrax,vikomall/pyrax,roelvv/pyrax,mandx/pyrax,EdLeafe/pyrax,briancurtin/pyrax,rackerlabs/heat-pyrax,opsdisk/pyrax,rackspace/pyrax,naemono/pyrax,0dataloss/pyrax,pratikmallya/pyrax