Add markers: fix coordinate list concatenation

[saerdnaer]

When saving a polygon to the database, the coordinates are suffixed with _ to split the individual values. As of now the javascript simply adds a lowerdash (_) after each value, but it only should use dashes to split the values and not at the end.

as of now:

Latitude: 50.8210729998_50.8222207292_50.82075_50.8196023064_
Longitude: -1.3146566479299509_-1.3128552788100478_-1.3105199999999968_-1.312321368049993_

should be:

Latitude: 50.8210729998_50.8222207292_50.82075_50.8196023064
Longitude: -1.3146566479299509_-1.3128552788100478_-1.3105199999999968_-1.312321368049993