exclude KeyType.Assigned properties from UPDATE'd columns

My project typically uses SQL Server tables that have PK's that are not identity columns, so I map them as KeyType.Assigned with ClassMapper.   While doing some testing today I noticed that Update<T> was generating SQL that was updating the PK columns, as well as including them in the where clause.  

For example, SQL Profiler showed me this:

```sql
exec sp_executesql N'UPDATE [ProdPosIss45] SET [ProdPosIss45].[StoreId] = @StoreId, [ProdPosIss45].[Upc] = @Upc, [ProdPosIss45].[MAN_PRC_FG] = @MAN_PRC_FG WHERE (([ProdPosIss45].[StoreId] = @StoreId_0) AND ([ProdPosIss45].[Upc] = @Upc_1))',N'@Upc varchar(4000),@StoreId smallint,@MAN_PRC_FG tinyint,@StoreId_0 smallint,@Upc_1 varchar(4000)',@Upc='0000000004012',@StoreId=2201,@MAN_PRC_FG=1,@StoreId_0=2201,@Upc_1='0000000004012'
```

And I was not expecting to see the PK fields being updated - I expected to only see them in the WHERE clause.

My change is to *exclude* any KeyType.Assigned properties from the UPDATE'd columns while still using them in a where clause.  Then SQL Profiler showed me this:

```sql
exec sp_executesql N'UPDATE [ProdPosIss45] SET [ProdPosIss45].[MAN_PRC_FG] = @MAN_PRC_FG WHERE (([ProdPosIss45].[StoreId] = @StoreId_0) AND ([ProdPosIss45].[Upc] = @Upc_1))',N'@MAN_PRC_FG tinyint,@StoreId_0 smallint,@Upc_1 varchar(4000)',@MAN_PRC_FG=1,@StoreId_0=2201,@Upc_1='0000000004011'
```

I was able to run all units tests (except for MySql) and all tests pass. 

This is the first PR I have ever submitted, hope it makes sense.