Fix memory leak in setting encodings

	There is a memory leak in Encoding.default_external= and
	Encoding.default_internal= because the duplicated name is not freed
	when overwriting.

	    10.times do
	      1_000_000.times do
	        Encoding.default_internal = nil
	      end

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

	Before:

	     25664
	     41504
	     57360
	     73232
	     89168
	    105056
	    120944
	    136816
	    152720
	    168576

	After:

	    9648
	    9648
	    9648
	    9680
	    9680
	    9680
	    9680
	    9680
	    9680
	    9680