Fix remove not exist key got exception related to Resource Not Found

[thohng]

To fix this issue: #37 when remove a not exist cache entry

Microsoft.Azure.Cosmos.CosmosException : Response status code does not indicate success: NotFound (404); Substatus: 0; ActivityId: 970d262f-dfcf-4b01-8ec1-1a1f25ee18bb; Reason: ({
  "Errors": [
    "Resource Not Found. Learn more: https://aka.ms/cosmosdb-tsg-not-found"
  ]
});
   at Microsoft.Azure.Cosmos.ResponseMessage.EnsureSuccessStatusCode()
   at Microsoft.Azure.Cosmos.CosmosResponseFactoryCore.ProcessMessage[T](ResponseMessage responseMessage, Func`2 createResponse)
   at Microsoft.Azure.Cosmos.CosmosResponseFactoryCore.CreateItemResponse[T](ResponseMessage responseMessage)
   at Microsoft.Azure.Cosmos.ContainerCore.DeleteItemAsync[T](CosmosDiagnosticsContext diagnosticsContext, String id, PartitionKey partitionKey, ItemRequestOptions requestOptions, CancellationToken cancellationToken)
   at Microsoft.Azure.Cosmos.ClientContextCore.RunWithDiagnosticsHelperAsync[TResult](CosmosDiagnosticsContext diagnosticsContext, Func`2 task)
   at Microsoft.Extensions.Caching.Cosmos.CosmosCache.RemoveAsync(String key, CancellationToken token)
....

[ghost]

All CLA requirements met.

[ealsur]

/azp run

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[ealsur]

/azp run

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[ealsur]

/azp run

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[ealsur]

@thohng Thanks for adding tests covering the scenario!