HTTPCLIENT-1507: clarified behavior of CloseableHttpResponse#close()

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpclient/trunk@1596521 13f79535-47bb-0310-9956-ffa450edef68
First attempt at making HEAD requests cacheable. Modified CacheableRequestPolicy, ResponseCachingPolicy  & CachedResponseSuitabilityChecker to permit HEAD requests being served from cache and added to cache if appropriate. Avoided any content length checking of an absent resource.
Merge branch 'trunk' of github.com:TJ-Cutajar/httpclient into trunk