Wrong union result in spherical coordinate system

Consider the test case:
```
namespace cartesian {
using point = boost::geometry::model::d2::point_xy;
using linestring = boost::geometry::model::linestring;
using polygon = boost::geometry::model::polygon;
using multipolygon = boost::geometry::model::multi_polygon;
using mpoint = boost::geometry::model::multi_point;
}

namespace spherical {
using point = boost::geometry::model::d2::point_xy >;
using linestring = boost::geometry::model::linestring;
using polygon = boost::geometry::model::polygon;
using multipolygon = boost::geometry::model::multi_polygon;
using mpoint = boost::geometry::model::multi_point;
const double earthRadiusMts = 6371000;
}

BOOST_AUTO_TEST_CASE(testUnionBug) {
auto p1wkt = "POLYGON((-78.4072265625001 43.06652924482626,-78.4072265625 43.06740063068311,-78.4063141178686 43.06653210403569,-78.4072265625001 43.06652924482626))";
auto p2wkt = "POLYGON((-78.55968743491499 43.06594969590624,-78.55036227331367 43.07380195109801,-78.53503704605811 43.08248347074284,-78.51769210872999 43.08880392487917,-78.49899564953199 43.09251971058174,-78.47966844278045 43.09348761253013,-78.46045580120891 43.09167037120638,-78.44209853911326 43.08713812460473,-78.42530412309867 43.08006566649393,-78.41071917768537 43.07072563376782,-78.40631359930124 43.06653210565861,-78.55968743491499 43.06594969590624))";

{
spherical::polygon p1,p2;
boost::geometry::read_wkt(p1wkt, p1);
boost::geometry::read_wkt(p2wkt, p2);

spherical::multipolygon out;
boost::geometry::union_(
p1,
p2,
out);
std::cout << "Spherical union: " << boost::geometry::wkt(out) << std::endl;
}
{
cartesian::polygon p1,p2;
boost::geometry::read_wkt(p1wkt, p1);
boost::geometry::read_wkt(p2wkt, p2);

cartesian::multipolygon out;
boost::geometry::union_(
p1,
p2,
out);
std::cout << "Cartesian union: " << boost::geometry::wkt(out) << std::endl;
}
}
```
when executed against [Boost 1.66](https://dl.bintray.com/boostorg/release/1.66.0/source/boost_1_66_0.tar.gz,) the output is:
```
Spherical union: MULTIPOLYGON()
Cartesian union: MULTIPOLYGON(((-78.5597 43.0659,-78.5504 43.0738,-78.535 43.0825,-78.5177 43.0888,-78.499 43.0925,-78.4797 43.0935,-78.4605 43.0917,-78.4421 43.0871,-78.4253 43.0801,-78.4107 43.0707,-78.4063 43.0665,-78.5597 43.0659)))
```
Now I assume that union can only be empty if both constituting geometries are empty and under this assumption i take it this a bug. I will continue trying to diagnose and put more info, but as diagnostics are time consuming, I am submitting the issue as i see it now.

This the same general problem domain myself and @vosst have been working on (and earlier raised https://github.com/boostorg/geometry/issues/470).