checkAvailability serialization exception if service not available

[ALobastov]

Steps:

1. In the messaging entry configuration page configure the Hours of Operation so the service is not available at the time of test
2. In the mobile app, call checkAvailability() method

Result: serialization exception:
Date: Mon, 28 Jun 2021 19:18:52 GMT
Server: nginx/1.19.2
Access-Control-Allow-Credentials: true
Access-Control-Allow-Methods: GET, POST
Content-Length: 23
Access-Control-Allow-Headers: Accept, Origin, Content-type, Authorization
Strict-Transport-Security: max-age=31536000; includeSubDomains
Content-Type: application/json;charset=utf-8
Connection: keep-alive

{"chat":"notAvailable"}
<------------------------

Failed to check availability: dataCorrupted(Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "chat", intValue: nil)], debugDescription: "Cannot initialize ContactCenterServiceChatAvailability from invalid String value notAvailable", underlyingError: nil))