nested_formatter compilation errors

[sergiud]

Using GCC 6.4.0 the API reference example

#include <fmt/format.h>

struct point {
  double x, y;
};

template <>
struct fmt::formatter<point> : nested_formatter<double> {
  auto format(point p, format_context& ctx) const {
    return write_padded(ctx, [=](auto out) {
      return format_to(out, "({}, {})", nested(p.x), nested(p.y));
    });
  }
};

int main() {
  fmt::print("[{:>20.2f}]", point{1, 2});
}

and the corresponding unit test produce compilation errors such as

error: cannot call member function 'fmt::v10::nested_view<T> fmt::v10::nested_formatter<T>::nested(const T&) const [with T = double]' without object
       return format_to(out, "({}, {})", nested(p.x), nested(p.y));

Reproducer: https://godbolt.org/z/6WoeMcs6a

[phprus]

Fix: #3810