DataObject.toString isn't connector aware

[anowell]

https://github.com/algorithmiaio/algorithmia-java/blob/e6d90c4e375ba868a5d082432412166b5cc9530b/src/main/java/com/algorithmia/data/DataObject.java#L68

If you use a dropbox or s3 connector, toString will return a URI like data://s3://foo/bar