Model.__init__ supporting same typing as assigment (#835)

* `Model.__init__` supporting same typing as assigment

* Update mypy_django_plugin/django/context.py

tests/typecheck/contrib/postgres/test_fields.yml

@@ -0,0 +1,19 @@
+-   case: union_combinable_reduced_to_non_union
+    main: |
+        from typing import List
+        from myapp.models import MyModel
+        array_val: List[int] = [1]
+        MyModel(array=array_val)  # E: Incompatible type for "array" of "MyModel" (got "List[int]", expected "Union[Sequence[str], Combinable]")
+        non_init = MyModel()
+        non_init.array = array_val  # E: Incompatible types in assignment (expression has type "List[int]", variable has type "Union[Sequence[str], Combinable]")
+    installed_apps:
+        - myapp
+    files:
+        -   path: myapp/__init__.py
+        -   path: myapp/models.py
+            content: |
+                from django.db import models
+                from django.contrib.postgres.fields import ArrayField
+
+                class MyModel(models.Model):
+                    array = ArrayField(base_field=models.TextField())