WSS over explicit proxies not implemented for asyncio

Steps to Reproduce

1. Use asyncio
2. Use the Component API
3. Pass a proxy in the transport for a wss router
4. Invoke sample code thusly: sample.py wss://myrouter.com/ws realm1 myproxy.com:3128

Expected Result

autobahn connects successfully

Actual Result

2019-09-27T14:10:02 connecting once using transport type "websocket" over endpoint "tcp"
2019-09-27T14:10:02 Exception in callback WebSocketAdapterProtocol._consume.<locals>.process(<Future finished result=None>) at /home/fferrell/src/proxy_example/venv/lib/python3.6/site-packages/autobahn/asyncio/websocket.py:106
handle: <Handle WebSocketAdapterProtocol._consume.<locals>.process(<Future finished result=None>) at /home/fferrell/src/proxy_example/venv/lib/python3.6/site-packages/autobahn/asyncio/websocket.py:106>
Traceback (most recent call last):
  File "/usr/lib/python3.6/asyncio/events.py", line 145, in _run
    self._callback(*self._args)
  File "/home/fferrell/src/proxy_example/venv/lib/python3.6/site-packages/autobahn/asyncio/websocket.py", line 110, in process
    self._dataReceived(data)
  File "/home/fferrell/src/proxy_example/venv/lib/python3.6/site-packages/autobahn/websocket/protocol.py", line 1213, in _dataReceived
    self.consumeData()
  File "/home/fferrell/src/proxy_example/venv/lib/python3.6/site-packages/autobahn/websocket/protocol.py", line 1232, in consumeData
    self.processProxyConnect()
  File "/home/fferrell/src/proxy_example/venv/lib/python3.6/site-packages/autobahn/websocket/protocol.py", line 3514, in processProxyConnect
    self.startTLS()
  File "/home/fferrell/src/proxy_example/venv/lib/python3.6/site-packages/autobahn/asyncio/websocket.py", line 230, in startTLS
    raise Exception("WSS over explicit proxies not implemented")
Exception: WSS over explicit proxies not implemented

Sample Code

import sys
from autobahn.asyncio.component import Component, run

proxy_host, proxy_port = sys.argv[3].split( ':' )

comp = Component(
    transports = dict(
        url = sys.argv[1],
        proxy = dict(
            host = proxy_host,
            port = int( proxy_port )
        ),
    ),
    realm = sys.argv[2],
)

@comp.on_join
def joined( session, details ):
    print( 'session ready' )

run( [ comp ] )

Notes

This works with twisted. Change the import in the sample code to autobahn.twisted.component and it connects as expected.

Start with autobahn/asyncio/websocket.py, especially WebSocketAdapterProtocol.startTLS, and follow the proxy path into autobahn/websocket/protocol.py at processProxyConnect. Compare the asyncio behavior with the working Twisted implementation and verify the sample.py WSS connection through an explicit proxy succeeds without the reported exception.