Merge branch 'master' of https://github.com/elasticsearch/elasticsearch-shield into doc-feedback

Conflicts:
	docs/structured/03-getting-started.asciidoc
	docs/structured/06-ldap.asciidoc
	docs/structured/authentication/message-authentication.asciidoc

Original commit: elastic/x-pack-elasticsearch@642beb53ca

src/main/java/org/elasticsearch/shield/SecurityFilter.java

@@ -129,7 +129,12 @@ public class SecurityFilter extends AbstractComponent {
 
         @Override
         public void process(RestRequest request, RestChannel channel, RestFilterChain filterChain) throws Exception {
-            filter.authenticate(request);
+
+            // CORS - allow for preflight unauthenticated OPTIONS request
+            if (request.method() != RestRequest.Method.OPTIONS) {
+                filter.authenticate(request);
+            }
+
             filterChain.continueProcessing(request, channel);
         }
     }

src/main/java/org/elasticsearch/shield/transport/SecuredTransportModule.java

@@ -53,6 +53,7 @@ public class SecuredTransportModule extends AbstractShieldModule.Spawn implement
         if (clientMode) {
             // no ip filtering on the client
             bind(N2NNettyUpstreamHandler.class).toProvider(Providers.<N2NNettyUpstreamHandler>of(null));
+            bind(TransportFilter.class).toInstance(TransportFilter.NOOP);
             return;
         }
 

src/main/java/org/elasticsearch/shield/transport/TransportFilter.java

@@ -13,6 +13,8 @@ import org.elasticsearch.transport.TransportResponse;
  */
 public interface TransportFilter {
 
+    static final TransportFilter NOOP = new Base();
+
     /**
      * Called just before the given request is about to be sent. Any exception thrown
      * by this method will stop the request from being sent.