improve the pretty printing of OrderedHash

I submitted the same ticket to Rails Guides, sorry.

It seems that the pretty printing of the instance of ActiveSupport::OrderedHash is broken.

>> pp [1, 1].group_by{|i| i}
{[1, [1, 1]]=>nil}
=> nil

After patching:

>> pp [1, 1].group_by{|i| i}
#<OrderedHash {1=>[1, 1]}>

and works correctly even if the hash is larger:

>> pp (1..20).group_by{|i| i % 3}
#<OrderedHash
{1=>[1, 4, 7, 10, 13, 16, 19],
 2=>[2, 5, 8, 11, 14, 17, 20],
 0=>[3, 6, 9, 12, 15, 18]}>
=> nil

Thanks,