Fix bug in hole in polygon algo

In trying to find a sample point, we iterate the ring's triplet coordinates and add add the second coord from the end to allow checking the last triplet. Previously the second coordinate was added by changing the main coords list in-place, but that messes with the point-in-polygon algorithm. Fixed by only iterating through the points instead of changing the coords list.
Fixes #205.

Author: karimbahgat

shapefile.py

@@ -237,9 +237,15 @@ def ring_sample(coords, ccw=False):
     The orientation of the ring is assumed to be clockwise, unless ccw
     (counter-clockwise) is set to True. 
     """
-    coords = tuple(coords) + (coords[1],) # add the second coordinate to the end to allow checking the last triplet
     triplet = []
-    for p in coords:
+    def itercoords():
+        # iterate full closed ring
+        for p in coords:
+            yield p
+        # finally, yield the second coordinate to the end to allow checking the last triplet
+        yield coords[1]
+        
+    for p in itercoords(): 
         # add point to triplet (but not if duplicate)
         if p not in triplet:
             triplet.append(p)