Update _doRequest signature for Encoder and Decoder

Author: kiike

src/Protocol/ClientDecoder.php

@@ -42,7 +42,7 @@ public function decode(string $function, string $response)
      *
      * @see \SoapClient::__doRequest()
      */
-    public function __doRequest($request, $location, $action, $version, $one_way = 0)
+    public function __doRequest($request, $location, $action, $version, $one_way = 0): ?string
     {
         // the actual result doesn't actually matter, just return the given result
         // this will be processed internally and will return the parsed result

src/Protocol/ClientEncoder.php

@@ -43,7 +43,7 @@ public function encode(string $name, array $args): RequestInterface
      *
      * @see \SoapClient::__doRequest()
      */
-    public function __doRequest($request, $location, $action, $version, $one_way = 0)
+    public function __doRequest($request, $location, $action, $version, $one_way = 0): ?string
     {
         $headers = array();
         if ($version === SOAP_1_1) {