Use coverage and add missing test

.coveragerc

@@ -0,0 +1,2 @@
+[run]
+source = jsignature

.gitignore

@@ -1 +1,3 @@
 *.pyc
+.coverage
+htmlcov

.travis.yml

@@ -10,5 +10,6 @@ env:
 install:
   - pip install -r requirements.txt --use-mirrors
   - pip install -q Django==$DJANGO_VERSION --use-mirrors
+  - pip install coverage
 
-script: python quicktest.py jsignature
+script: coverage run quicktest.py jsignature

jsignature/tests/mixins.py

@@ -48,3 +48,11 @@ class JSignatureFieldsMixinTest(SimpleTestCase):
         i.save()
         i = JSignatureTestModel.objects.get(pk=i.pk)
         self.assertEqual(date.today(), i.signature_date.date())
+
+        # If sinature is set to None, it must be the same for signature_date
+        i = JSignatureTestModel(signature=signature_value)
+        i.save()
+        i.signature = None
+        i.save()
+        i = JSignatureTestModel.objects.get(pk=i.pk)
+        self.assertIsNone(i.signature_date)