RFR [12] 8218662: Allow 204 responses with Content-Length:0

[Chris Hegarty]

This is a code review request for a late fix for JDK 12, to address a
what could be perceived as a potential regression.

Post 8211437 [1] the HTTP Client now disallows 204 responses with a
response body. It appears that this change may have been a little too
strict, by disallowing any response with a Content-Length header. It
seems reasonable to allow the special case where a content length of
zero is indicated. This is a small relaxing of the change for 8211437,
while keeping its most fundamental aspects.

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8218662
Webrev:
  https://cr.openjdk.java.net/~chegar/8218662/webrev.00/

-Chris.

[1] https://bugs.openjdk.java.net/browse/JDK-8211437