Fix string conversion under Clang

[wancw]

Explicit conversion to std::string by static_cast<> causes compilation error when using Clang (3.5).
(Casting to const std::string doesn't work either.)

This PR fixed it by removing const from the casting operator.
And I think it also makes casting operator more consistent with get<>() method.

[coveralls]

Coverage remained the same when pulling 36f6199 on wancw:fix_clang_string_casting into bd05c7a on nlohmann:master.

[nlohmann]

Thanks a lot for fixing that!