Support compressed data downloads

Support downloading of compressed attachments if that
attachment can be compressed as determined by the
att_encoding_info=true doc query parameter
I've got some logic for this in getAttachmentStream as well. Maybe take the logic out of these two places and put it into Attachment.java? A method like encodingFromString should look like: null or "Plain" → Plain, gzip → Gzip, all others → throw IllegalArgumentException (it's possible, but unlikely that there will be more encodings in future)