Fixed #28188 -- Fixed crash when pickling model fields.

Regression in d2a26c1a90.

Thanks Adam Alton for the report and test, and Adam Johnson for
suggesting the fix.

tests/model_fields/tests.py

@@ -1,3 +1,5 @@
+import pickle
+
 from django import forms
 from django.db import models
 from django.test import SimpleTestCase, TestCase
@@ -76,6 +78,13 @@ class BasicFieldTests(TestCase):
         self.assertIsNotNone(f1)
         self.assertNotIn(f2, (None, 1, ''))
 
+    def test_field_instance_is_picklable(self):
+        """Field instances can be pickled."""
+        field = models.Field(max_length=100, default='a string')
+        # Must be picklable with this cached property populated (#28188).
+        field._get_default
+        pickle.dumps(field)
+
 
 class ChoicesTests(SimpleTestCase):