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

0 files changed, 0 insertions, 0 deletions