Tests: update test object creation for Django 1.8

Several of the field tests previously assigned a related test model
instance before saving it::

    mock_tag = MockTag(name='primary')
    mock = MockModel()
    mock.tag = mock_tag

Django 1.8 now validates this dodgy practice and throws an error.

This commit simply changes it to use `create()` so the mock_tag will
have a pk before assignment.

Author: acdha

test_haystack/test_fields.py

@@ -27,7 +27,8 @@ def test_prepare(self):
         self.assertEqual(author.prepare(mock), u'daniel')
 
         # Do a lookup through the relation.
-        mock_tag = MockTag(name='primary')
+        mock_tag = MockTag.objects.create(name='primary')
+
         mock = MockModel()
         mock.tag = mock_tag
         tag_name = CharField(model_attr='tag__name')
@@ -41,7 +42,8 @@ def test_prepare(self):
         self.assertEqual(author.prepare(mock), u'')
 
         # Simulate failed lookups.
-        mock_tag = MockTag(name='primary')
+        mock_tag = MockTag.objects.create(name='primary')
+
         mock = MockModel()
         mock.tag = mock_tag
         tag_slug = CharField(model_attr='tag__slug')
@@ -84,7 +86,8 @@ def test_prepare(self):
         self.assertEqual(author.prepare(mock), u'daniel')
 
         # Do a lookup through the relation.
-        mock_tag = MockTag(name='primary')
+        mock_tag = MockTag.objects.create(name='primary')
+
         mock = MockModel()
         mock.tag = mock_tag
         tag_name = NgramField(model_attr='tag__name')
@@ -98,7 +101,8 @@ def test_prepare(self):
         self.assertEqual(author.prepare(mock), u'')
 
         # Simulate failed lookups.
-        mock_tag = MockTag(name='primary')
+        mock_tag = MockTag.objects.create(name='primary')
+
         mock = MockModel()
         mock.tag = mock_tag
         tag_slug = NgramField(model_attr='tag__slug')
@@ -141,7 +145,8 @@ def test_prepare(self):
         self.assertEqual(author.prepare(mock), u'daniel')
 
         # Do a lookup through the relation.
-        mock_tag = MockTag(name='primary')
+        mock_tag = MockTag.objects.create(name='primary')
+
         mock = MockModel()
         mock.tag = mock_tag
         tag_name = EdgeNgramField(model_attr='tag__name')
@@ -155,7 +160,8 @@ def test_prepare(self):
         self.assertEqual(author.prepare(mock), u'')
 
         # Simulate failed lookups.
-        mock_tag = MockTag(name='primary')
+        mock_tag = MockTag.objects.create(name='primary')
+
         mock = MockModel()
         mock.tag = mock_tag
         tag_slug = EdgeNgramField(model_attr='tag__slug')
@@ -196,7 +202,8 @@ def test_prepare(self):
         self.assertEqual(pk.prepare(mock), 1)
 
         # Simulate failed lookups.
-        mock_tag = MockTag(name='primary')
+        mock_tag = MockTag.objects.create(name='primary')
+
         mock = MockModel()
         mock.tag = mock_tag
         tag_count = IntegerField(model_attr='tag__count')