Ignore case of S3 custom meta-data headers when getting Object

As per the HTTP/1.1 RFC, the message headers should be case-insensitive

http://tools.ietf.org/html/rfc2616#section-4.2

In line with the S3 documentation, the user-defined meta data keys should always be lower-cased:

http://docs.aws.amazon.com/AmazonS3/latest/dev/UsingMetadata.html