Fix " Illegal target characters" when connecting to URL with zero-len…

…gth path

This regressed in 0233e21 (version 0.9.0) due
to different behavior between yarl and urllib.parse.urlsplit on a URL
without a path component like "wss://ws.kraken.com". yarl makes this `/`
while urlsplit makes it ''. h11 expects `/` as specified in the quoted
RFC.

tests/test_connection.py

@@ -284,6 +284,11 @@ async def test_client_open_url(echo_server):
     async with open_websocket_url(url) as conn:
         assert conn.path == RESOURCE + '?foo=bar'
 
+    # Zero-length path becomes '/' in the HTTP Request Target.
+    url = 'ws://{}:{}'.format(HOST, echo_server.port)
+    async with open_websocket_url(url) as conn:
+        assert conn.path == ''
+
 
 async def test_client_open_invalid_url(echo_server):
     with pytest.raises(ValueError):

trio_websocket/_impl.py

@@ -708,7 +708,14 @@ def __init__(self, stream, wsproto, *, host=None, path=None,
         self._max_message_size = max_message_size
         self._reader_running = True
         if wsproto.client:
-            self._initial_request = Request(host=host, target=path,
+            if path:
+                target = path
+            else:
+                # RFC 7230, Section 5.3.1:
+                # If the target URI's path component is empty, the client MUST
+                # send "/" as the path within the origin-form of request-target.
+                target = '/'
+            self._initial_request = Request(host=host, target=target,
                 subprotocols=client_subprotocols,
                 extra_headers=client_extra_headers or [])
         else: