Added Convert.ToDateTime as a supported method for translation #1297 - #1298

Maybe it would have been best if I didn't created a pull request so soon. I could have asked the same referencing the commit on my fork without creating a pull request.

@TanielianVB no worries about creating a PR!

Yeah, it actually is that simple (though a test would also be good). The reason this hasn't been done is that unlike Convert.ToInt32, Convert.ToDateTime would not work on the database in the same way as it would work in C#: the date format strings accepted by PostgreSQL and by .NET ToDateTime are very different. Let's wait for some feedback in #1297 before doing anything.

I think that if a dev uses Convert.ToDateTime he has to know in what SQL the query will be translated. But, we can also add functions like EF.Functions.ILike to the TO_DATE and TO_TIMESTAMP functions.

The general principle in EF is for methods to behave in the same way when translated to the server, as they would when executed on the client - this is why Convert.ToDateTime is somewhat problematic... But you're right that in any case we can provide something on EF.Functions.

Already made all requested changes.