Fix hardcoded US date time format in unit tests

[jonorossi]

I'm an Australian and our general date format is dd/mm/yyyy like many other countries, so 10/30/2013 is an illegal date to parse. I've fixed the unit tests to remove this ambiguity.

If you'd prefer I can change them to using the DateTime and DateTimeOffset constructors.

RethinkDb.Newtonsoft.Test.DatumConversion.DateTimeOffsetTests.ser_deser_a_datetimeoffset : System.FormatException : String was not recognized as a valid DateTime.
at System.DateTimeOffset.Parse (System.String input, IFormatProvider formatProvider, DateTimeStyles styles) [0x00000] in <filename unknown>:0
at System.DateTimeOffset.Parse (System.String input, IFormatProvider formatProvider) [0x00000] in <filename unknown>:0
at System.DateTimeOffset.Parse (System.String input) [0x00000] in <filename unknown>:0
at RethinkDb.Newtonsoft.Test.DatumConversion.DateTimeOffsetTests.ser_deser_a_datetimeoffset () [0x00000] in <filename unknown>:0
at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00000] in <filename unknown>:0

[mfenniak]

Thanks! I probably would like using the constructors slightly more, but, since this fixes the issue and it's only unit test code, I'm happy to merge it.