Add tests for to_integer(), fix to_integer()

gsl/gsl_byte

@@ -103,7 +103,7 @@ inline constexpr byte operator~(byte b) noexcept { return byte(~static_cast<unsi
 template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
 inline constexpr IntegerType to_integer(byte b) noexcept
 {
-    return {b};
+    return static_cast<IntegerType>(b);
 }
 
 template<bool E, typename T>