StringIO#set_encoding

static VALUE
strio_set_encoding(int argc, VALUE *argv, VALUE self)
{
    rb_encoding* enc;
    struct StringIO *ptr = StringIO(self);
    VALUE ext_enc, int_enc, opt;

    argc = rb_scan_args(argc, argv, "11:", &ext_enc, &int_enc, &opt);

    if (NIL_P(ext_enc)) {
        enc = rb_default_external_encoding();
    }
    else {
        enc = rb_to_encoding(ext_enc);
    }
    ptr->enc = enc;
    if (WRITABLE(self)) {
        rb_enc_associate(ptr->string, enc);
    }

    return self;
}