client: s/v2Prefix/DefaultV2KeysPrefix/
client: s/v2URL/v2KeysURL/
client: copy v2KeysPrefix to httpClient

It's poor form to muck with a global variable. Make a copy when the
httpClient object is instantiated to make httpClient.SetPrefix safe.
Merge pull request #1377 from bcwaldon/client-cleanup

Make httpClient.SetPrefix safer