Add routing middleware (#1497)

* Add routing middleware

Factor out starlette BaseHTTPMiddleware

Fix exceptions for starlette < 0.19

Fix docstring formatting

Rename middleware/base.py to abstract.py

Rework routing middleware

* Clean up abstract API docstrings

* Move connexion context into extensions

* Allow empty middleware list

tests/test_middleware.py

@@ -0,0 +1,44 @@
+import pytest
+from connexion.middleware import ConnexionMiddleware
+from connexion.middleware.routing import CONNEXION_CONTEXT
+from starlette.datastructures import MutableHeaders
+
+from conftest import SPECS, build_app_from_fixture
+
+
+class TestMiddleware:
+    """Middleware to check if operation is accessible on scope."""
+
+    def __init__(self, app):
+        self.app = app
+
+    async def __call__(self, scope, receive, send):
+        operation_id = scope['extensions'][CONNEXION_CONTEXT]['operation_id']
+
+        async def patched_send(message):
+            if message["type"] != "http.response.start":
+                await send(message)
+                return
+
+            message.setdefault("headers", [])
+            headers = MutableHeaders(scope=message)
+            headers["operation_id"] = operation_id
+
+            await send(message)
+
+        await self.app(scope, receive, patched_send)
+
+
+@pytest.fixture(scope="session", params=SPECS)
+def middleware_app(request):
+    middlewares = ConnexionMiddleware.default_middlewares + [TestMiddleware]
+    return build_app_from_fixture('simple', request.param, middlewares=middlewares)
+
+
+def test_routing_middleware(middleware_app):
+    app_client = middleware_app.app.test_client()
+
+    response = app_client.post("/v1.0/greeting/robbe")
+
+    assert response.headers.get('operation_id') == 'fakeapi.hello.post_greeting', \
+        response.status_code