Memory leak when duplicating identhash

	[Bug #20145]

	Before this commit, both copy_compare_by_id and hash_copy will create a
	copy of the ST table, so the ST table created in copy_compare_by_id will
	be leaked.

	    h = { 1 => 2 }.compare_by_identity

	    10.times do
	      1_000_000.times do
	        h.select { false }
	      end

	      puts `ps -o rss= -p #{$$}`
	    end

	Before:

	    110736
	    204352
	    300272
	    395520
	    460704
	    476736
	    542000
	    604704
	    682624
	    770528

	After:

	    15504
	    16048
	    16144
	    16256
	    16320
	    16320
	    16752
	    16752
	    16752
	    16752
	---
	 hash.c                 | 10 +++++++++-
	 test/ruby/test_hash.rb | 10 ++++++++++
	 2 files changed, 19 insertions(+), 1 deletion(-)