Add tests for Lambert azimuthal extrema

These tests produce the LamberAzimuthalEqualArea CRS with central
latitudes at the poles to ensure the extreme cases work as required.

lib/cartopy/tests/crs/test_lambert_azimuthal_equal_area.py

@@ -19,6 +19,7 @@
 
 import numpy as np
 from numpy.testing import assert_almost_equal
+import pytest
 
 import cartopy.crs as ccrs
 
@@ -59,3 +60,10 @@ def test_offset(self):
         assert crs_offset.proj4_init == expected
         assert tuple(np.array(crs.x_limits) + 1234) == crs_offset.x_limits
         assert tuple(np.array(crs.y_limits) - 4321) == crs_offset.y_limits
+
+    @pytest.mark.parametrize("latitude", [-90, 90])
+    def test_extrema(self, latitude):
+        crs = ccrs.LambertAzimuthalEqualArea(central_latitude=latitude)
+        expected = ('+ellps=WGS84 +proj=laea +lon_0=0.0 +lat_0={} '
+                    '+x_0=0.0 +y_0=0.0 +no_defs'.format(latitude))
+        assert crs.proj4_init == expected