As discovered by @felipegb94, there are missing overloads to the
`GenericValue::AddMember<T>` template function, taking an explicit
`GenericValue&` as a name and accepting arbitrary primitive values.

This patch adds the missing overloads. The `StringRefType` overload
is needed to disambiguate the addition of a string literal as
value.

Some tests are added to `TEST(Value, Object)` in `valuetest.cpp`.