'melt' can preserve attributes as well, like 'dcast' does now

[arunsrinivasan]

Now that #688 is implemented, to be consistent, we'll also try to retain attributes in melt wherever possible.

Note that melt.data.table used to retain attributes, which was changed to be consistent with melt from reshape2 at the time. But since reshape2's melt now also retain attributes, this change would be much better.

[arunsrinivasan]

This commit 07b90de fixes it. melt now retains attributes wherever possible.