Allow primitive value type results to be cast to Nullable<T> counterparts.

Author: mrworkman

src/LinqToExcel/Query/ExcelQueryExecutor.cs

@@ -133,8 +133,11 @@ protected bool ShouldBuildResultObjectMapping<T>(object firstResult, QueryModel
                                                  typeof (SumResultOperator)
                                              };
 
+            var resultType = firstResult?.GetType();
+
             return (firstResult != null &&
-                    firstResult.GetType() != typeof(T) &&
+                    resultType != typeof(T) &&
+                    resultType != Nullable.GetUnderlyingType(typeof(T)) &&
                     !queryModel.ResultOperators.Any(x => ignoredResultOperators.Contains(x.GetType())));
         }