fix php7 regression

Author: mevdschee

api.include.php

@@ -2268,7 +2268,7 @@ public function withoutAttribute($attribute): ServerRequestInterface
      */
     class Stream implements StreamInterface
     {
-        function __toString() {}
+        function __toString():string { return ""; }
 
         /** @var resource|null A resource reference */
         private $stream;

api.php

@@ -2268,7 +2268,7 @@ public function withoutAttribute($attribute): ServerRequestInterface
      */
     class Stream implements StreamInterface
     {
-        function __toString() {}
+        function __toString():string { return ""; }
 
         /** @var resource|null A resource reference */
         private $stream;

build.php

@@ -109,7 +109,7 @@ function run(string $base, array $dirs, string $filename, array $ignore, array $
 ];
 
 $replaces = [
-    'use StreamTrait;' => 'function __toString() {}'
+    'use StreamTrait;' => 'function __toString():string { return ""; }'
 ];
 
 $directories = ['vendor/nyholm', 'src'];