Throw an exception when asked to Delete<MyType>(null) rather than trying to as...

Throw an exception when asked to Delete<MyType>(null) rather than trying to as in Npgsql it will delete the whole table.

Dapper.Contrib/SqlMapperExtensions.cs

@@ -255,6 +255,9 @@ private static string GetTableName(Type type)
         /// <returns>true if deleted, false if not found</returns>
         public static bool Delete<T>(this IDbConnection connection, T entityToDelete, IDbTransaction transaction = null, int? commandTimeout = null) where T : class
         {
+			if (entityToDelete == null)
+				throw new ArgumentException("Cannot Delete null Object", "entityToDelete");
+
             var type = typeof(T);
 
             var keyProperties = KeyPropertiesCache(type);