Add head request to GN proxy and manage a 404
validate the url in Onlineres window if it is http protocol
send error codestatus for head request
proxy : delete doHead and manage code status
Merge pull request #286 from fgravin/checkLinkOnlinesrc-PR

Check url link in online resource window
Merge remote-tracking branch 'upstream/develop' into spring-data-jpa

Conflicts:
	core/src/main/java/org/geonetwork/http/proxy/HttpProxyServlet.java