Enhanced Polygon Test

[Wulfheart]

I added some more tests to check a polygon.

The new output is:

--- FAIL: TestPolygonNotClosed (0.00s)
    d:\code\golang-geo\polygon_test.go:164: Nope! The polygon is not closed!
--- FAIL: TestPolygonSimple (0.00s)
    d:\code\golang-geo\polygon_test.go:204: x:  0 	y:  0 	Expected:  true 	Got:  false
    d:\code\golang-geo\polygon_test.go:204: x:  1 	y:  1 	Expected:  true 	Got:  false
FAIL
FAIL	github.com/Wulfheart/golang-geo	0.694s
Error: Tests failed.

As you may see additionally to #66 also the IsClosed() method is broken. I will open an issue for it.

[Wulfheart]

Stale.