Force IndexFormat value to lowercase

[Mohamad-Sobhie]

when using the index format as below, the code will not show any error, yet it will not log to elasticsearch because the indexing format should be lowered-case. This should be handled somewhere.

    var logger = new LoggerConfiguration()
            .WriteTo.Elasticsearch(new ElasticsearchSinkOptions(new Uri("http://localhost:9200"))
            {
                IndexFormat = "TESTING",
            }).CreateLogger();

[mivano]

Is fixed in dev branch, will go to master soon. Tx.