Review return-value converter

Return-value converter seems to work now only for explicit type-initialized variables:
```
std::string out = encode<codec::base64>(in.begin(), in.end());
```

The same construction fails if the variable was initialized earlier:
```
std::string str = "123";
str = encode<codec::hex>(str.begin(), str.end());
```

This leads to ambiguous ```operator=``` selection.