Expand test suite

[vikram7]

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!

[bitwalker]

I'd gladly merge a PR for more tests! I'm keenly aware of how lacking tests for this library are, and would love to remedy that, sadly I've been cramped on time as of late. Any help is greatly appreciated!

[tessi]

FYI: I've added a PR testing Timex.Ecto.DateTimeWithTimezone.cast/1 -> #30