multiple authorization for one app

It will be nice , to test it automatically , but i think it is impossible because we must obtain 2 tokens manually
> Запрещается отправлять запрос (открывать страницу) непосредственно из приложения, поскольку по правилам платежной системы логин, пароль и платежный пароль пользователя допускается вводить только на страницах сервиса «Яндекс.Деньги».

I tested this code manually , and it works.

So , for detailed explanation , this test will be like this: 
```
  it "should initialize with multiple instances" do
    
    VCR.use_cassette "initialize twice" do

      url1 = YandexMoney::Wallet.build_obtain_token_url(
        CLIENT_ID,
        REDIRECT_URI,
        "account-info operation-history",
        "first instance"
      )

      url2 = YandexMoney::Wallet.build_obtain_token_url(
        CLIENT_ID,
        REDIRECT_URI,
        "account-info",
        "second instance"
      )

      # go to first url, accept , get first access_token

      # go to second url, accept, get second access_token

      api1 = YandexMoney::Wallet.new(ACCESS_TOKEN)

      api2 = YandexMoney::Wallet.new(ACCESS_TOKEN_2)

      expect {
        api1.operation_history
        api2.account_info
      }.not_to raise_error

    end
  end
```
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