Implemented pillar.flask_extra.ensure_schema(url)

This function ensures that the URL has the correct schema, given the
app configuration. This is required because the Flask instance can sit
behind an SSL-terminating proxy like HAProxy and not know that it is
reachable via HTTPS.

tests/test_flask_extra.py

@@ -33,3 +33,54 @@ class FlaskExtraTest(unittest.TestCase):
         self.assertEqual(201, resp.status_code)
         self.assertNotIn('Vary', resp.headers)
         self.assertEqual('nah', resp.data.decode())
+
+
+class EnsureSchemaTest(unittest.TestCase):
+    def test_ensure_schema_http(self):
+        import pillar.flask_extra
+
+        suffix = '://user:password@hostname/some-path/%2Fpaththing?query=abc#fragment'
+
+        app = flask.Flask(__name__)
+        app.config['PREFERRED_URL_SCHEME'] = 'http'
+        with app.app_context():
+            for scheme in ('http', 'https', 'ftp', 'gopher'):
+                self.assertEqual(
+                    f'http{suffix}',
+                    pillar.flask_extra.ensure_schema(f'{scheme}{suffix}'))
+
+    def test_ensure_schema_https(self):
+        import pillar.flask_extra
+
+        suffix = '://user:password@hostname/some-path/%2Fpaththing?query=abc#fragment'
+
+        app = flask.Flask(__name__)
+        app.config['PREFERRED_URL_SCHEME'] = 'https'
+        with app.app_context():
+            for scheme in ('http', 'https', 'ftp', 'gopher'):
+                self.assertEqual(
+                    f'https{suffix}',
+                    pillar.flask_extra.ensure_schema(f'{scheme}{suffix}'))
+
+    def test_no_config(self):
+        import pillar.flask_extra
+
+        suffix = '://user:password@hostname/some-path/%2Fpaththing?query=abc#fragment'
+
+        app = flask.Flask(__name__)
+        app.config.pop('PREFERRED_URL_SCHEME', None)
+        with app.app_context():
+            self.assertEqual(
+                f'https{suffix}',
+                pillar.flask_extra.ensure_schema(f'gopher{suffix}'))
+
+    def test_corner_cases(self):
+        import pillar.flask_extra
+
+        app = flask.Flask(__name__)
+        app.config['PREFERRED_URL_SCHEME'] = 'https'
+        with app.app_context():
+            self.assertEqual('', pillar.flask_extra.ensure_schema(''))
+            self.assertEqual('/some/path/only', pillar.flask_extra.ensure_schema('/some/path/only'))
+            self.assertEqual('https://hostname/path',
+                             pillar.flask_extra.ensure_schema('//hostname/path'))