RFR: 8209178: Proxied HttpsURLConnection doesn't send BODY when retrying POST request

[Julia Boes]

Hi,

This fix addresses an issue in HttpClient when retrying a POST request 
over proxied HTTPS. In this case, the request body is stored in a 
PosterOutputStream poster, which is set to null in 
sun/net/www/protocol/http/HttpURLConnection::sendCONNECTRequest after 
the first request.

The fix addresses this issue by storing the current state of poster in a 
local variable in a new method HttpClient::checkTunneling.

Note: The system property 'sun.net.http.retryPost' defaults to true in 
HttpClient line 180 but the test is run explicitly for both options for 
readability.

Bug: https://bugs.openjdk.java.net/browse/JDK-8209178

Webrev: 
http://cr.openjdk.java.net/~jboes/webrevs/8209178/webrev.00/index.html

The test case was provided by the submitter of the original bug, Arrault 
Fabien.


Regards,

Julia