json parser fails to handle non-BMP unicode escapes

``` cpp
#include <cpprest/json.h>

int main(int argc, char *argv[]) {
    ucout << web::json::value::parse(U("{\"test\":\"\\ud83c\\uddee\\ud83c\\udde9\"}"))
                 .at(U("test"))
                 .as_string()
          << std::endl;
}
```

terminate called after throwing an instance of 'std::range_error'
  what():  UTF-16 string is missing low surrogate
