Fixed #22962 -- Default values for ArrayField with migrations.

Fields normally try to force the default value to a string. As
translatable strings are not valid default values for ArrayField, we can
remove this behaviour which was causing issues with some migrations.

Thanks to @schinckel for the report.

tests/postgres_tests/array_default_migrations/0001_initial.py

@@ -0,0 +1,64 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import models, migrations
+import django.contrib.postgres.fields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='CharArrayModel',
+            fields=[
+                ('id', models.AutoField(verbose_name='ID', serialize=False, auto_created=True, primary_key=True)),
+                ('field', django.contrib.postgres.fields.ArrayField(models.CharField(max_length=10), size=None)),
+            ],
+            options={
+            },
+            bases=(models.Model,),
+        ),
+        migrations.CreateModel(
+            name='DateTimeArrayModel',
+            fields=[
+                ('id', models.AutoField(verbose_name='ID', serialize=False, auto_created=True, primary_key=True)),
+                ('field', django.contrib.postgres.fields.ArrayField(models.DateTimeField(), size=None)),
+            ],
+            options={
+            },
+            bases=(models.Model,),
+        ),
+        migrations.CreateModel(
+            name='IntegerArrayModel',
+            fields=[
+                ('id', models.AutoField(verbose_name='ID', serialize=False, auto_created=True, primary_key=True)),
+                ('field', django.contrib.postgres.fields.ArrayField(models.IntegerField(), size=None)),
+            ],
+            options={
+            },
+            bases=(models.Model,),
+        ),
+        migrations.CreateModel(
+            name='NestedIntegerArrayModel',
+            fields=[
+                ('id', models.AutoField(verbose_name='ID', serialize=False, auto_created=True, primary_key=True)),
+                ('field', django.contrib.postgres.fields.ArrayField(django.contrib.postgres.fields.ArrayField(models.IntegerField(), size=None), size=None)),
+            ],
+            options={
+            },
+            bases=(models.Model,),
+        ),
+        migrations.CreateModel(
+            name='NullableIntegerArrayModel',
+            fields=[
+                ('id', models.AutoField(verbose_name='ID', serialize=False, auto_created=True, primary_key=True)),
+                ('field', django.contrib.postgres.fields.ArrayField(models.IntegerField(), size=None, null=True, blank=True)),
+            ],
+            options={
+            },
+            bases=(models.Model,),
+        ),
+    ]

tests/postgres_tests/array_default_migrations/0002_integerarraymodel_field_2.py

@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import models, migrations
+import django.contrib.postgres.fields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('postgres_tests', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='integerarraymodel',
+            name='field_2',
+            field=django.contrib.postgres.fields.ArrayField(models.IntegerField(), default=[], size=None),
+            preserve_default=False,
+        ),
+    ]