Added ability to set key as (not)nullable via convention

Author: hazzik

src/FluentNHibernate.Testing/ConventionsTests/ApplyingToModel/HasManyCollectionConventionTests.cs

@@ -192,6 +192,22 @@ public void ShouldSetPropertyRefProperty()
             VerifyModel(x => x.Key.PropertyRef.ShouldEqual("xxx"));
         }
 
+        [Test]
+        public void KeyNullableShouldSetModelValue()
+        {
+            Convention(x => x.KeyNullable());
+
+            VerifyModel(x => x.Key.NotNull.ShouldBeFalse());
+        }
+
+        [Test]
+        public void KeyNotNullableShouldSetModelValue()
+        {
+            Convention(x => x.Not.KeyNullable());
+
+            VerifyModel(x => x.Key.NotNull.ShouldBeTrue());
+        }
+
         [Test]
         public void ShouldSetTableNameProperty()
         {

src/FluentNHibernate/Conventions/Instances/CollectionInstance.cs

@@ -122,6 +122,12 @@ public void Subselect(string subselect)
             mapping.Set(x => x.Subselect, Layer.Conventions, subselect);
         }
 
+        public void KeyNullable()
+        {
+            mapping.Key.Set(x => x.NotNull, Layer.Conventions, !nextBool);
+            nextBool = true;
+        }
+
         public void Table(string tableName)
         {
             mapping.Set(x => x.TableName, Layer.Conventions, tableName);

src/FluentNHibernate/Conventions/Instances/ICollectionInstance.cs

@@ -43,5 +43,6 @@ public interface ICollectionInstance : ICollectionInspector
         new void OrderBy(string orderBy);
         new void Sort(string sort);
         void Subselect(string subselect);
+        void KeyNullable();
     }
 }