URI-encode forwarded location header in proxy

Right now there are situations in which ElasticSearch puts characters in
the character code range between 128 and 255 into the `Location` header.
This leads to an exception when trying to pass on that header through
the hapi proxy in versions before 15.0.0, because it validates that only
US-ASCII characters are used in headers.

To work around that situation, the `Location` header is encoded using
`encodeURI()` for now.

Closes #8705

src/core_plugins/elasticsearch/lib/create_proxy.js

@@ -20,7 +20,14 @@ function createProxy(server, method, route, config) {
         xforward: true,
         timeout: server.config().get('elasticsearch.requestTimeout'),
         onResponse: function (err, responseFromUpstream, request, reply) {
-          reply(err, responseFromUpstream);
+          const upstreamLocation = responseFromUpstream.headers.location;
+          const response = reply(err, responseFromUpstream);
+
+          // Workaround for #8705 until hapi has been updated to >= 15.0.0
+          if (upstreamLocation) {
+            delete responseFromUpstream.headers.location;
+            response.location(encodeURI(upstreamLocation));
+          }
         }
       }
     },