add round trip test for project/ interpolate

test/geos_capi/line_string_test.rb

@@ -18,6 +18,14 @@ class TestLineString < ::Test::Unit::TestCase # :nodoc:
         def setup
           @factory = ::RGeo::Geos.factory
         end
+
+        def test_project_interpolate_round_trip
+          point =  @factory.point(2,2)
+          line_string = @factory.line_string([ [0,0], [5,5] ].map { |x,y| @factory.point(x,y) })
+          location = line_string.project_point point
+          interpolated_point = line_string.interpolate_point location
+          assert_equal point, interpolated_point
+        end
       end
     end
   end