Use HeadObject for statRequest

[jcharum]

Use HeadObject instead of GetObject for handling statRequests. Besides being a better semantic match, it fixes #15.

[josh-newman]

Does startGetObjectRequest still need to construct newInfo?

[jcharum]

Does startGetObjectRequest still need to construct newInfo?

I don't think so, but I'm only 80% sure, as I have little experience with this code. My reasoning goes like this:

1. startGetObjectRequest will only call newInfo if f.info == nil.
2. startGetObjectRequest is only called by handleRead.
3. handleRead is only called to handle a readRequest.
4. readRequest is only issued in (*s3Reader).Read.
5. The only way to get an *s3Reader is by (*s3File).Reader. It only does this if f.mode == readonly.
6. The only way to get a readonly *s3File is by (*s3Impl).Open.
7. (*s3Impl).Open will only return successfully if the handleStat sends a non-error response, via runRequest and statRequest.
8. handleStat only sends a non-error response after populating f.info.
9. Therefore, f.info != nil in startGetObjectRequest, and it will not call newInfo.

I'm not really sure how to thoroughly test removal of this code right now, so I don't want to add it to this PR. The change in this PR seems much less risky.

Also, I looked through the history, and I think the f.info == nil case became obsolete in 3cf2bd3, which introduced the statRequest on (*s3Impl).Open.