return 'connection: close' header in response

According to https://www.w3.org/Protocols/rfc2616/rfc2616-sec8.html a
server should return the 'connection: close' header in the response when
a request has that header. Resolves #720

tests/test_main.py

@@ -8,6 +8,34 @@
 from uvicorn.main import Server
 
 
+def test_return_close_header():
+    class App:
+        def __init__(self, scope):
+            if scope["type"] != "http":
+                raise Exception()
+
+        async def __call__(self, receive, send):
+            await send({"type": "http.response.start", "status": 200, "headers": []})
+            await send({"type": "http.response.body", "body": b"", "more_body": False})
+
+    class CustomServer(Server):
+        def install_signal_handlers(self):
+            pass
+
+    config = Config(app=App, loop="asyncio", limit_max_requests=1)
+    server = CustomServer(config=config)
+    thread = threading.Thread(target=server.run)
+    thread.start()
+    while not server.started:
+        time.sleep(0.01)
+    response = requests.get("http://127.0.0.1:8000", headers={"connection": "close"})
+    thread.join()
+    assert response.status_code == 200
+    assert (
+        "connection" in response.headers and response.headers["connection"] == "close"
+    )
+
+
 def test_run():
     class App:
         def __init__(self, scope):

uvicorn/protocols/http/h11_impl.py

@@ -24,6 +24,7 @@ def _get_status_phrase(status_code):
 STATUS_PHRASES = {
     status_code: _get_status_phrase(status_code) for status_code in range(100, 600)
 }
+CLOSE_HEADER = (b"connection", b"close")
 
 HIGH_WATER_LIMIT = 65536
 
@@ -445,6 +446,8 @@ async def send(self, message):
 
             status_code = message["status"]
             headers = self.default_headers + message.get("headers", [])
+            if CLOSE_HEADER in self.scope["headers"] and CLOSE_HEADER not in headers:
+                headers = headers + [CLOSE_HEADER]
 
             if self.access_log:
                 self.access_logger.info(

uvicorn/protocols/http/httptools_impl.py

@@ -25,6 +25,7 @@ def _get_status_line(status_code):
 STATUS_LINE = {
     status_code: _get_status_line(status_code) for status_code in range(100, 600)
 }
+CLOSE_HEADER = (b"connection", b"close")
 
 HIGH_WATER_LIMIT = 65536
 
@@ -443,6 +444,8 @@ async def send(self, message):
 
             status_code = message["status"]
             headers = self.default_headers + list(message.get("headers", []))
+            if CLOSE_HEADER in self.scope["headers"] and CLOSE_HEADER not in headers:
+                headers = headers + [CLOSE_HEADER]
 
             if self.access_log:
                 self.access_logger.info(