Fix schema generation with OneToOneFields (#81)

* Fix: OneToOneRel, used by OneToOneField doesn't have help_text nor primary_key attributes, thus breaking OpenAPISchemaGenerator; use hasattr() as safe-guard.
* Fix: use getattr() with a default value instead of hasattr() + acessing the value
* Add: 'people' app that breaks drf_yasg without previous commits
* Update tests/references.yaml + run isort and flake8
* Fix: set on_delete for Person.identity as Django-2+ requires it

testproj/people/migrations/0001_initial.py

@@ -0,0 +1,32 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.10 on 2018-03-18 16:22
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    initial = True
+
+    dependencies = [
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Identity',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('firstName', models.CharField(max_length=30, null=True)),
+                ('lastName', models.CharField(max_length=30, null=True)),
+            ],
+        ),
+        migrations.CreateModel(
+            name='Person',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('Identity', models.OneToOneField(on_delete=django.db.models.deletion.CASCADE, related_name='person', to='people.Identity')),
+            ],
+        ),
+    ]

testproj/people/migrations/0002_auto_20180318_1704.py

@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.10 on 2018-03-18 17:04
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('people', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='person',
+            name='Identity',
+            field=models.OneToOneField(on_delete=django.db.models.deletion.PROTECT, related_name='person', to='people.Identity'),
+        ),
+    ]