Only set creator when creating task (#80)

* Only set creator when creating task

* allow blank form input, use clean func to keep old val

* remove unneeded created_date assignment

* add test

todo/migrations/0011_auto_20190724_1130.py

@@ -0,0 +1,20 @@
+# Generated by Django 2.1.8 on 2019-07-24 11:30
+
+from django.conf import settings
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('todo', '0010_attachment'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='task',
+            name='created_by',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, related_name='todo_created_by', to=settings.AUTH_USER_MODEL),
+        ),
+    ]