fix several bugs in findSingleAttibute (#980)

* add test case for distinct on id property

* suggested fix for "select distinct id"

* add test case for distinct with fetch

* FIX: .setDistinct(true) can be used in conjunction with fetch() now

* add test case for distinct with beans that have a disriminator column

* FIX: discriminator column is only read if also Id is read.

* fine tuned the test case

* added test cases for findSingleAttributeList without distinct

* improved fix to support also findSingleAttribute without distinct

* ADD: Bonus test case - assertion not yet verified

src/main/java/io/ebeaninternal/server/querydefn/OrmQueryDetail.java

@@ -440,7 +440,7 @@ public class OrmQueryDetail implements Serializable {
     }
   }
 
-  private boolean hasSelectClause() {
+  public boolean hasSelectClause() {
     return baseProps.hasSelectClause();
   }