Fixes timestamp serialization issue (#170)

[snechaev]

This should fix #170. The changes are:

• Using invariant culture to serialize timestamps
• Corresponding change to the Assert expression (the expression relates on the timestamp serialization options).

[Inumedia]

Can you confirm that this works with .netcore?

[snechaev]

Yes, it should work on .netcore. According to MSDN, DateTime.ToString(String, IFormatProvider) and DateTime.ToString(IFormatProvider) overloads are available starting from .net core 1.0 as well as CultureInfo.InvariantCulture property (see https://docs.microsoft.com/en-us/dotnet/api/system.globalization.cultureinfo.invariantculture?view=netcore-1.0 and https://docs.microsoft.com/en-us/dotnet/api/system.datetime.tostring?view=netcore-1.0#System_DateTime_ToString_System_String_System_IFormatProvider_)

[gpailler]

The AppVeyor build is ok so it should work properly with .net core