Spline tangnet fix for offset workplane (#293)

Fixes #292

tests/test_cadquery.py

@@ -545,6 +545,13 @@ class TestCadQuery(BaseTest):
         path1 = Workplane("XZ").spline(pts[1:], includeCurrent=True).val()
         self.assertAlmostEqual(path.Length(), path1.Length())
 
+        # test tangents and offset plane
+        pts = [(0, 0), (-1, 1), (-2, 0), (-1, 0)]
+        tangents = [(0, 1), (1, 0)]
+
+        path2 = Workplane("XY", (0, 0, 10)).spline(pts, tangents=tangents)
+        self.assertAlmostEqual(path2.val().tangentAt(0).z, 0)
+
     def testSweep(self):
         """
         Tests the operation of sweeping a wire(s) along a path