Fix build with Clang6

/usr/include/c++/v1/utility:639:1: note: candidate function not viable: 1st

argument ('const std::string' (aka 'const basic_string<char,
char_traits<char>, allocator<char> >')) would lose const qualifier

make_pair(_T1&& t1, _T2&& t2)

Patch includes more explanation.

Reported by: linimon