Test and fix for Point.is_collinear coincident point error.

Previously, if the first two points passed to Point.is_collinear were
coincident, the method would always return True.  This is fixed by converting
the point list to a set to remove identical points.
Merge pull request #2158 from Tempel/coincident_collinear

Test and fix for Point.is_collinear coincident point error.