FIX: if regrid shape is passed, we need to remove it from the super call

The regrid doesn't do anything, so we should warn the user that we are
getting rid of it and continuing on.

lib/cartopy/mpl/geoaxes.py

@@ -1282,6 +1282,11 @@ def imshow(self, img, *args, **kwargs):
 
         if (transform is None or transform == self.transData or
                 same_projection and inside_bounds):
+            if "regrid_shape" in kwargs:
+                warnings.warn("ignoring regrid_shape because it doesn't do anything "
+                              "when working in the same projection. To avoid this "
+                              "warning, remove the 'regrid_shape' keyword argument.")
+                kwargs.pop("regrid_shape")
             result = super().imshow(img, *args, **kwargs)
         else:
             extent = kwargs.pop('extent', None)

lib/cartopy/tests/mpl/test_images.py

@@ -139,6 +139,16 @@ def test_imshow_wrapping():
     assert ax.get_xlim() == (-180, 180)
 
 
+def test_imshow_arguments():
+    """Smoke test for imshow argument passing in the fast-path"""
+    ax = plt.axes(projection=ccrs.PlateCarree())
+    # Set the regrid_shape parameter to ensure it isn't passed to Axes.imshow()
+    # in the fast-path call to super()
+    with pytest.warns(UserWarning, match="ignoring regrid_shape"):
+        ax.imshow(np.random.random((10, 10)), transform=ccrs.PlateCarree(),
+              extent=(-180, 180, -90, 90), regrid_shape=500)
+
+
 def test_imshow_rgba():
     # tests that the alpha of a RGBA array passed to imshow is set to 0
     # instead of masked