Fix msvc /fp:fast test failure

[fo40225]

fix #767

[cdunn2001]

../src/test_lib_json/main.cpp:2408:28: error: ‘isnan’ was not declared in this scope
     JSONTEST_ASSERT(isnan(n));

This change is fine, but it needs to pass TravisCI too. Maybe we need #include <cmath>, or std::isnan().

[cdunn2001]

Very nice!