This should probably not be visible to the user unless they use the
non-typesafe `Gson.toJson(Object, Type)` where unrelated number types can
be used, or when malformed generic containers are used. For example a
`List<Byte>` containing a Float.

This change also has the advantage of avoiding `JsonWriter.value(Number)`
for primitive type adapters. That method has some overhead because it needs
to make sure that the value is a valid JSON number. However, for primitive
numbers this check is redundant.