fix SNI for transparent mode - #1638

In transparent mode host is set with the target server ip.
Attribute flow.request.host is used as SNI while mitmproxy is
initiating TLS handshake, so it should be set with the pretty_host.

examples/sslstrip.py

@@ -16,6 +16,7 @@ def request(flow):
     if flow.request.pretty_host in secure_hosts:
         flow.request.scheme = 'https'
         flow.request.port = 443
+        flow.request.host = flow.request.pretty_host
 
 
 def response(flow):