Review return-value converter

[nemothenoone]

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.