Improve RelatedField and callable default handling

- callable default values will now be properly called 
  - PrimaryKeyRelatedField and SlugRelatedField will now return an appropriate type based on the relation model's Field
  - mock views now have a request object bound even when public is True

testproj/snippets/migrations/0002_auto_20171205_0505.py

@@ -1,27 +0,0 @@
-# Generated by Django 2.0 on 2017-12-05 04:05
-
-from django.conf import settings
-from django.db import migrations, models
-import django.db.models.deletion
-
-
-class Migration(migrations.Migration):
-
-    dependencies = [
-        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
-        ('snippets', '0001_initial'),
-    ]
-
-    operations = [
-        migrations.AddField(
-            model_name='snippet',
-            name='owner',
-            field=models.ForeignKey(default='', on_delete=django.db.models.deletion.CASCADE, related_name='snippets', to=settings.AUTH_USER_MODEL),
-            preserve_default=False,
-        ),
-        migrations.AlterField(
-            model_name='snippet',
-            name='code',
-            field=models.TextField(help_text='code model help text'),
-        ),
-    ]