Expand test suite

The current test suite contains a single test for Timex.Ecto.Time.cast/1 when the input is a string. Other tests are likely needed here:

• Timex.Ecto.Time.cast/1 when input is binary
• Timex.Ecto.Time.cast/1when input is a timestamp
• Timex.Ecto.Time.cast/1 when input is a map like %{"calendar" => . . . }
• Timex.Ecto.Time.cast/1 when input is time
• Timex.Ecto.Time.load/1
• Timex.Ecto.Time.dump/1
• Timex.Ecto.DateTime.cast/1 functions here
• Timex.Ecto.DateTimeWithTimezone .cast/1, .dump/1, .load/1 functions here
• Timex.Ecto.Date functions here

I'm happy to put up a PR for these and others that you might think are necessary!