Update return by value threshold in gsl::not_null (#1205)

- Allow returning by value for types that are not greater than two pointers in size

include/gsl/pointers

@@ -54,7 +54,7 @@ namespace details
     // Copied from cppfront's implementation of the CppCoreGuidelines F.16 (https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rf-in)
     template<typename T>
     using value_or_reference_return_t = std::conditional_t<
-                                            sizeof(T) < 2*sizeof(void*) && std::is_trivially_copy_constructible<T>::value,
+                                            sizeof(T) <= 2*sizeof(void*) && std::is_trivially_copy_constructible<T>::value,
                                             const T,
                                             const T&>;