Allow control over date header addition

This allows for Hypercorn to be used behind servers/proxies that add
this header as well.

src/hypercorn/config.py

@@ -76,6 +76,7 @@ class Config:
     h2_max_concurrent_streams = 100
     h2_max_header_list_size = 2**16
     h2_max_inbound_frame_size = 2**14 * OCTETS
+    include_date_header = True
     include_server_header = True
     keep_alive_timeout = 5 * SECONDS
     keyfile: Optional[str] = None
@@ -267,7 +268,9 @@ class Config:
         return sockets
 
     def response_headers(self, protocol: str) -> List[Tuple[bytes, bytes]]:
-        headers = [(b"date", format_date_time(time()).encode("ascii"))]
+        headers = []
+        if self.include_date_header:
+            headers.append((b"date", format_date_time(time()).encode("ascii")))
         if self.include_server_header:
             headers.append((b"server", f"hypercorn-{protocol}".encode("ascii")))