In certain STLs, `std::vector<bool>::reference` is just `bool&`.
Prepending `const` causes the following warning:
```
include/nlohmann/detail/conversions/to_json.hpp:271:42: error: 'const' qualifier on reference type 'std::vector<bool>::reference' (aka 'bool &') has no effect [-Werror,-Wignored-reference-qualifiers]
```
This PR fixes the problem.