boost::icl::interval_map::operator+= : entry doesn't get added if value is zero.

When inserting an entry into a `boost::icl::interval_map`, the entry is not inserted if the value is 0. I am guessing this is a bug that happens when the value evaluates to a boolean false. note: using VS2017, C++14. Issue is present in 1.59 - 1.69 (including C++17 for 1.69)

```

#include
#include
#include

using namespace std;
typedef pair float_pair;

boost::icl::interval_map makeIntervalMap(const vector& intervals)
{
boost::icl::interval_map result;
int intervalCount = 0;

for (float_pair entry : intervals) {
result += make_pair(
boost::icl::interval::right_open(entry.first, entry.second),
intervalCount++
);
}

return result;
}

const vector intervals = {
make_pair(200.0f,250.0f),
make_pair(100.0f,199.0f),
make_pair(0.0f,50.0f),
};

int main()
{
boost::icl::interval_map m = makeIntervalMap(intervals);

for (auto it = m.begin(); it != m.end(); ++it) {
cout << it->first.lower() << ", " << it->first.upper() << endl;
}
}
```

In the same code, the interval_map has one fewer element than it should, with the interval corresponding to value 0 missing, Changing it to use a ones-based index works as a workaround. And if I remove the post-increment and try to assign every interval the value zero, none of them get added.