Manually convert true/false to "true"/"false" before http_build_query()

`http_build_query()` converts true/false to 0/1. Was not a problem previously, as `refresh`
didn't accept any actual values.  But now that it accepts `wait_for`, etc the conversion is bad.

We need to manually convert true/false to "true"/"false" strings now.

src/Elasticsearch/Connections/Connection.php

@@ -294,6 +294,14 @@ private function wrapHandler(callable $handler, LoggerInterface $logger, LoggerI
     private function getURI($uri, $params)
     {
         if (isset($params) === true && !empty($params)) {
+            array_walk($params, function (&$value, &$key) {
+                if ($value === true) {
+                    $value = 'true';
+                } else if ($value === false) {
+                    $value = 'false';
+                }
+            });
+
             $uri .= '?' . http_build_query($params);
         }