FIX: close the path on GeoSpines around the boundary

The GeoSpine class surrounds the entire region and thus should be closed.
Previously, there would be a small gap at the corner of closure due
to the capstyle of the lines being "butt" by default. To avoid any
capstyle issues, we can just set the path to be closed.

lib/cartopy/mpl/geoaxes.py

@@ -262,7 +262,6 @@ def __init__(self, axes, **kwargs):
         self._original_path = mpath.Path(np.empty((0, 2)))
         kwargs.setdefault('clip_on', False)
         super().__init__(axes, 'geo', self._original_path, **kwargs)
-        self.set_capstyle('butt')
 
     def set_boundary(self, path, transform):
         self._original_path = path
@@ -272,6 +271,7 @@ def set_boundary(self, path, transform):
     def _adjust_location(self):
         if self.stale:
             self._path = self._original_path.clip_to_bbox(self.axes.viewLim)
+            self._path = mpath.Path(self._path.vertices, closed=True)
 
     def get_window_extent(self, renderer=None):
         # make sure the location is updated so that transforms etc are

lib/cartopy/tests/mpl/test_images.py

@@ -69,7 +69,7 @@ def test_web_tiles():
 
 @pytest.mark.natural_earth
 @pytest.mark.network
-@pytest.mark.mpl_image_compare(filename='image_merge.png', tolerance=0.01)
+@pytest.mark.mpl_image_compare(filename='image_merge.png', tolerance=0.03)
 def test_image_merge():
     # tests the basic image merging functionality
     tiles = []