Include x-amz-security-token in s3 during signing

To quote the documentation:
```
If you are signing your request using temporary security credentials (see Making Requests),
you must include the corresponding security token in your request by adding the
x-amz-security-token header.
```

It wasn't included in headers before, and s3 couldn't be accessed with
the temporary token. Luckily this commit fixes that.
Merge pull request #36 from libc/s3_iam_fix_signing

Include x-amz-security-token in s3 during signing