support return Tuple

[xieyufei1220]

I want to pass the specified fields and then return tuple. It's ok with linq to db but error with EF 6. The exception is : Only parameterless constructors and initializers are supported in LINQ to Entities.

code:

async Task Main()
{
	Pick<(string, string)>("x=>new {x.AuthorCode,x.AuthorName}").Dump();  
}

public List<T> Pick<T>(string selector)
{
	return this.Author.Select<T>(selector).ToList();
}

can be improved to support return tuple as linq to db?

[StefH]

Related to #491