Fix ArgumentError invalid byte sequence in UTF-8 error

### Before

```
options[:shop_name] = '野碕商店'
gateway = GmoKonbiniGateway.new(options) # ArgumentError invalid byte sequence in UTF-8
```

### After

```
options[:shop_name] = '野碕商店'
gateway = GmoKonbiniGateway.new(options) # OK
```