Update Mercator projection attributes (#163)

* Update Mercator projection attributes

* remove empty test_mercator file

src/wrf/projection.py

@@ -76,12 +76,9 @@ if cartopy_enabled():
                 else:
                     xlimits[0] = -xlimits[0]
 
-            self._xlimits = tuple(xlimits)
-            self._ylimits = tuple(limits[..., 1])
-
             # Compatibility with cartopy >= 0.17
-            self._x_limits = self._xlimits
-            self._y_limits = self._ylimits
+            self._x_limits = tuple(xlimits)
+            self._y_limits =  tuple(limits[..., 1])
 
             self._threshold = np.diff(self.x_limits)[0] / 720