Fix small file requests

[bgdavidx]

AWS S3 returns response code 200 instead of 206 for small files. This allows s3gof3r to fetch these small files too without this error output:

2015/11/03 11:37:26 error on attempt 0: retrying chunk: 0, error: 200: ""
2015/11/03 11:37:26 error on attempt 1: retrying chunk: 0, error: 200: ""
2015/11/03 11:37:26 error on attempt 2: retrying chunk: 0, error: 200: ""
2015/11/03 11:37:27 error on attempt 3: retrying chunk: 0, error: 200: ""
2015/11/03 11:37:28 error on attempt 4: retrying chunk: 0, error: 200: ""
2015/11/03 11:37:29 error on attempt 5: retrying chunk: 0, error: 200: ""

[maxheadroom]

We were suffering from this problem as well. We've asked AWS to revert the change. That seem to be in effect now. The problem for us only occured in US-West-2 region.
However, this patch is useful as we probably have to expect a 200 Response for such cases. I'm still waiting for confirmation from AWS whether they will keep the 206 responses or eventually move to the 200 responses for small files.