When passing in a rvalue reference, compiler
considers it ambiguous between std::string and
std::string&&. Making one of them take a lvalue
reference makes compilers correctly pick the right
one depending on whether the passed in value binds
to a rvalue or lvalue reference.
Github-Pull: arun11299/cpp-subprocess#110
Rebased-From: 2d8a8eebb03e509840e2c3c755d1abf32d930f33