uaiohttpclient: Only http: protocol is supported, fail predictably for others.

uaiohttpclient/uaiohttpclient.py

@@ -48,6 +48,8 @@ def request_raw(method, url):
     except ValueError:
         proto, dummy, host = url.split("/", 2)
         path = ""
+    if proto != "http:":
+        raise ValueError("Unsupported protocol: " + proto)
     reader, writer = yield from asyncio.open_connection(host, 80)
     # Use protocol 1.0, because 1.1 always allows to use chunked transfer-encoding
     # But explicitly set Connection: close, even though this should be default for 1.0,