Allow support of initializer lists in Zserio optional

[mikir]

Zserio Optional assignment and emplace() are not working on initializer lists. The reason is that template parameters cannot be deduced. Example:

typedef zserio::Optional<std::vector<int>> OptionalVectorInt;
OptionalVectorInt v;    
v = {4};

The previous example is not compilable and returns an error "couldn’t deduce template parameter ‘U’"

Consider to use something like

BasicOptional& operator=(T&& value)
{
    emplace(std::forward<T>(value));
    return *this;
}

Reported by @reinco.