Fix the row index display for GroupBy/GatherBy (#90)

* Fix the row index display for GroupBy/GatherBy

Now the row index will be shown in numerical order, not in permutation.

* Update io.jl

Implemented by enumerate()

Author: Kexi002

src/abstractdataset/io.jl

@@ -268,10 +268,10 @@ function _show(io::IO, ::MIME"text/html", ds::Union{AbstractDataset, GroupBy, Ga
         write(io, mainmsg)
 
     end
-    for row in _get_perms(ds)[1:mxrow]
+    for (i, row) in enumerate(_get_perms(ds)[1:mxrow])
         write(io, "<tr>")
         if rowid === nothing
-            write(io, "<th>$row</th>")
+            write(io, "<th>$i</th>")
         else
             write(io, "<th>$rowid</th>")
         end