polymorphism bug

Signed-off-by: Emmanuel Mathot <emmanuel.mahot@gmail.com>
Merge branch 'master' of github.com:Terradue/DotNetGeoJson

Signed-off-by: Emmanuel Mathot <emmanuel.mahot@gmail.com>

Conflicts:
	Terradue.GeoJson.Tests/GmlTest.cs
	Terradue.GeoJson/Terradue/GeoJson/Feature/MultiLineStringFeature.cs