Convert Set to Array before using as header

This is needed in cases where a table rendered using an Array of Hashes has more than `maxcols` columns. The error is `NoMethodError` with `Set#[]` on line 83, where header is indexed into.

Author: CGamesPlay

lib/iruby/formatter.rb

@@ -61,7 +61,8 @@ def table(obj, maxrows: 15, maxcols: 15, **options)
           rows << row
         end
 
-        if header = keys
+        if keys
+          header = keys.to_a
           keys.merge(0...array_size)
         else
           keys = 0...array_size