Deligate equality to other if conversion to Unit fails. (#150)

* Punt equality to other if conversion to Unit fails.

* Add unit tests for not implemented equality.

Author: mrshannon

cf_units/__init__.py

@@ -1731,7 +1731,10 @@ def __eq__(self, other):
             True
 
         """
-        other = as_unit(other)
+        try:
+            other = as_unit(other)
+        except ValueError:
+            return NotImplemented
 
         # Compare category (i.e. unknown, no_unit, etc.).
         if self.category != other.category:

cf_units/tests/test_unit.py

@@ -667,6 +667,10 @@ def test_unknown_no_unit(self):
         v = Unit('no_unit')
         self.assertNotEqual(u, v)
 
+    def test_not_implemented(self):
+        u = Unit('meter')
+        self.assertFalse(u == {})
+
 
 class Test_non_equality(unittest.TestCase):
 
@@ -695,6 +699,10 @@ def test_no_unit(self):
         u = Unit('no_unit')
         self.assertFalse(u != 'no_unit')
 
+    def test_not_implemented(self):
+        u = Unit('meter')
+        self.assertNotEqual(u, {})
+
 
 class Test_convert(unittest.TestCase):