Add test for climatic_summary when there's no summaries provided

[dannyparsons]

@HawardKetoyoMsatsi Lily found a bug here #67 which wasn't picked up by the tests I had written because I didn't include a test example where the summaries parameter is not provided e.g. this example below.

climatic_summary(data = daily_niger, date_time = "date", station = "station_name",
                              elements = c("rain", "tmax"))

Could you add a test for this case? It can be any call to climatic_summary where summaries is not provided. In this case, it uses the default summary which is dplyr::n().