Support http.proxy and other system properties by default.

Only when HttpComponents >= 4.2, via SystemDefaultHttpClient
Upgrade HttpComponents to 4.2.5 from 4.1.2.
Improve URL fragment handling.

Subclasses can conditionally remove propagating the fragment. And we don't bother testing for it as user agents shouldn't send it any way.  HC changed the way it does it as of 4.3 to not send it.