Update IIsEntityDecider to use ExpressionsHelper.TryGetMappedType

[maca88]

Extracted from #2316 and will allow the following query to work:

db.Query<Animal>().Select(o => o.FatherOrMother.SerialNumber).ToList();

// Not mapped property
public Animal FatherOrMother => Father ?? Mother;

The current logic detects FatherOrMother as a mapped property and a join is added, which prevents #2316 from support the above query. By using ExpressionsHelper.TryGetMappedType prevents FatherOrMother to be detected as a mapped property.

[fredericDelaporte]

Can you add a test case for the kind of query this change allows support?

[maca88]

Can you add a test case for the kind of query this change allow support?

By just updating IIsEntityDecider the mentioned query in the first post does not execute, because SelectClauseHqlNominator will still nominate the not mapped property as evaluatable. With the last commit I added a simple fix for SelectClauseHqlNominator issue and a test case.