Fixed a regression introduced by HTTPCLIENT-1545

* Initially a SEC_E_DOWNGRADE_DETECTED was thrown but this one is not available on Windows XP and tests never pass. Rather use SEC_E_TARGET_UNKNOWN.
* Hostnames must be in lower case when a SPN is provided

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpclient/trunk@1639679 13f79535-47bb-0310-9956-ffa450edef68