Fix bug that prevents returning Flask.Response's

which
https://flask-rebar.readthedocs.io/en/latest/quickstart/basics.html#marshaling
promises should be supported:

  > if response_body_schema is None, the return value must be a return
  > value that Flask supports, e.g. a string or a Flask.Response object

flask_rebar/rebar.py

@@ -71,7 +71,7 @@ def _unpack_view_func_return_value(rv):
 
     This imitates Flask's own `Flask.make_response` method.
 
-    :param Optional[tuple] rv: (body, status, headers), (body, status), or (body, headers)
+    :param rv: (body, status, headers), (body, status), (body, headers), or body
     :return: (body, status, headers)
     :rtype: tuple
     """
@@ -146,11 +146,19 @@ def wrapped(*args, **kwargs):
         if not response_body_schema:
             return rv
 
-        data, status_code, headers = _unpack_view_func_return_value(rv)
+        if isinstance(rv, current_app.response_class):
+            schema = response_body_schema[rv.status_code]
+            # The schema may be set to None to bypass marshaling (e.g. for 204 responses).
+            if schema is None:
+                return rv
+            # Otherwise, ensure the response body conforms to the promised schema.
+            schema.loads(rv.data)  # May raise ValidationError.
+            return rv
 
+        data, status_code, headers = _unpack_view_func_return_value(rv)
         schema = response_body_schema[status_code]  # May raise KeyError.
 
-        # The schema may be declared as None to bypass marshaling (e.g. for 204 responses).
+        # The schema may be set to None to bypass marshaling (e.g. for 204 responses).
         if schema is None:
             return response(
                 data=data, status_code=status_code, headers=headers, mimetype=mimetype

tests/test_rebar.py

@@ -11,7 +11,7 @@
 import unittest
 
 import marshmallow as m
-from flask import Flask
+from flask import Flask, make_response
 
 from flask_rebar import messages
 from flask_rebar import HeaderApiKeyAuthenticator, SwaggerV3Generator
@@ -248,6 +248,46 @@ def update_foo(foo_uid):
         )
         self.assertEqual(resp.status_code, 400)
 
+    def test_flask_response_instance_interop_body_matches_schema(self):
+        rebar = Rebar()
+        registry = rebar.create_handler_registry()
+        schema = FooUpdateSchema()
+
+        @registry.handles(rule="/foo", method="PUT", response_body_schema=schema)
+        def foo():
+            return make_response(({"name": "foo"}, {"foo": "bar"}))
+
+        app = create_rebar_app(rebar)
+        resp = app.test_client().put(path="/foo")
+        self.assertEqual(resp.status_code, 200)
+        self.assertEqual(resp.headers["foo"], "bar")
+
+    def test_flask_response_instance_interop_body_does_not_match_schema(self):
+        rebar = Rebar()
+        registry = rebar.create_handler_registry()
+        schema = FooUpdateSchema()
+
+        @registry.handles(rule="/foo", method="PUT", response_body_schema=schema)
+        def foo():
+            return make_response({"does not match": "foo schema"})
+
+        app = create_rebar_app(rebar)
+        resp = app.test_client().put(path="/foo")
+        self.assertEqual(resp.status_code, 500)
+
+    def test_flask_response_instance_interop_no_schema(self):
+        rebar = Rebar()
+        registry = rebar.create_handler_registry()
+
+        @registry.handles(rule="/foo", response_body_schema={302: None})
+        def foo():
+            return make_response(("Redirecting", 302, {"Location": "http://foo.com"}))
+
+        app = create_rebar_app(rebar)
+        resp = app.test_client().get(path="/foo")
+        self.assertEqual(resp.status_code, 302)
+        self.assertEqual(resp.headers["Location"], "http://foo.com")
+
     def test_validate_query_parameters(self):
         rebar = Rebar()
         registry = rebar.create_handler_registry()