Fixed #27860 -- Dropped varchar_pattern_ops/text_pattern_ops index before altering char/text field in PostgreSQL.

Thanks Tim Graham for the review.

tests/schema/models.py

@@ -17,6 +17,13 @@ class Author(models.Model):
         apps = new_apps
 
 
+class AuthorCharFieldWithIndex(models.Model):
+    char_field = models.CharField(max_length=31, db_index=True)
+
+    class Meta:
+        apps = new_apps
+
+
 class AuthorTextFieldWithIndex(models.Model):
     text_field = models.TextField(db_index=True)