Use upper-case "Host" header

Closes #614

Author: polyfractal

src/Elasticsearch/Connections/Connection.php

@@ -156,7 +156,7 @@ public function performRequest($method, $uri, $params = null, $body = null, $opt
             'uri'         => $this->getURI($uri, $params),
             'body'        => $body,
             'headers'     => array_merge([
-                'host'  => [$this->host]
+                'Host'  => [$this->host]
             ], $this->headers)
         ];