Support aggregate functions with more then one argument

[stowellt]

Hi,

Entity Framework v6.3.0 has support for aggregate functions with more then one argument. Could Function.DbModel.cs be updated to support that?

[kemsky]

@stowellt, did you try that yourself? I tried to implement STRING_AGG but it fails inside ef6 telling that argument count is not correct.

See dotnet/ef6#2090