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_find_encoding(ext_enc);
        if (!enc) {
            rb_io_enc_t convconfig;
            int oflags;
            rb_io_mode_t fmode;
            VALUE vmode = rb_str_append(rb_str_new_cstr("r:"), ext_enc);
            rb_io_extract_modeenc(&vmode, 0, Qnil, &oflags, &fmode, &convconfig);
            enc = convconfig.enc2;
        }
    }
    ptr->enc = enc;
    if (!NIL_P(ptr->string) && WRITABLE(self) && !str_chilled_p(ptr->string)) {
        rb_enc_associate(ptr->string, enc);
    }

    return self;
}