Added eight point BoundingSphereTest

Added eight point BoundingSphereTest that works with XNA test
environment but fails with monogame test environment
fix BoundingSphere.CreateFromPoints
Merge branch 'develop' of https://github.com/stampergames/MonoGame into BoundingSphere
Merge pull request #3162 from nkast/BoundingSphere

fix BoundingSphere.CreateFromPoints
Merge back from develop
Specify type
Merge branch 'master' into vorbisfile-cs