Skip the ETag header check in responce while using SSE-C encrpytion of S3

AWS S3 provides the new feature - [server side  encrpytion with customer keys](http://docs.aws.amazon.com/AmazonS3/latest/dev/ServerSideEncryptionCustomerKeys.html). I can add three headers - "x-amz-server-side​-encryption​-customer-algorithm,x-amz-server-side​-encryption​-customer-key,x-amz-server-side​-encryption​-customer-key-MD5" to use the Key.set_contents_from_filename and upload the file. However, the ETag check in response stops the operation. 

According to the AWS document, this header takes no effect while using the new SSE-C encryption. So this fix is to skip this check. "x-amz-server-side​-encryption​-customer-algorithm" is [one header in the response](http://docs.aws.amazon.com/AmazonS3/latest/API/RESTObjectPUT.html), which I use to determine if it is SSE-C encryption.