Fixing the canopy check to assure there is air above the grow block

Hello,

I actually didn't realize you were still active on Bukkit. If I had, I would have filed a bug request before writing this patch.

The current implementation was destroying decorative hedges which have leaves placed directly upon the ground. This code fix will short circuit the growAt method if it detects a canopy which doesn't have air directly above the grow block. I do not think anyone will want to configure this behavior, but I might be wrong.

Thanks!
Ben