enable_mobile! for auth

Enable ability to setup https://m.sp-money.yandex.ru for authorization:
```ruby
YandexMoney::Wallet.enable_mobile!
```
It is needed for mobile apps, see [here](https://tech.yandex.ru/money/doc/dg/reference/request-access-token-docpage/) or [here](https://tech.yandex.ru/money/doc/dg/reference/obtain-access-token-docpage/) and etc in authorization api block.
Surrounding space missing in default value assignment.