[s3] Do not prepend metadata keys if they start with x-amz

According to
http://docs.aws.amazon.com/AmazonS3/latest/API/RESTObjectPUT.html

to enable redirect on an S3 object it needs to have
'x-amz-website-redirect' metadata key with new location value.

When creating an object like so:

```ruby

bucket.object['foo'].write('', metadata: { "x-amz-website-redirect" =>
"http://example.com" }, acl: :public_read )
```

`x-amz-website-redirect` gets prepended with `x-amz-meta-` prefix thus
breaking the redirect feature.

This also affects any other meta data key which starts with 'x-amz'.