fix: panic on convert to string on fillSliceFromString()

[cjf8134]

fix: 修复fillSliceFromString()方法中mapValue 强转string后的panic 错误

[kevwan]

need unit test.

[cjf8134]

// 测试json参数
func TestUnmarshalJsonReaderArray(t *testing.T) {
payload := "{"id": 123}"
var res struct {
ID []string json:"id"
}
reader := strings.NewReader(payload)
err := UnmarshalJsonReader(reader, &res)
assert.Nil(t, err)
assert.Equal(t, 2, len(res.ID))
}

[kevwan]

This test case passed even without this PR.