RFR 8213189: Make restricted headers in HTTP Client configurable and remove Date by default

[Michael McMahon]

Could I get the following change reviewed please?

We've reduced the number of restricted headers to the minimum and
made those settable via a networking property (with health warning)

http://cr.openjdk.java.net/~michaelm/8213189/webrev.1/

This change will require a CSR for the new property.

Thanks,

Michael