Fix passing the correct content-type to Amazon S3

This pull request fixes passing the correct content-type to Amazon S3 adapter.

Uploading without the fix to S3 works, but sets default content-type: "application/octet-stream" for all uploaded files.

The fix passes the correct content type to the Gaufrette File class as metadata.
Also fixed the Replicate adapter, to support Metadata setting on child objects.

The issue mentioned was also asked on sonata-users:

https://groups.google.com/d/msg/sonata-users/3NiGqBpVL1g/Txm8LQslA3cJ