Fix GridFS providers

Author: GromNaN

tests/GridFS/BucketFunctionalTest.php

@@ -214,7 +214,7 @@ public function testDownloadToStreamShouldRequireDestinationStream($destination)
         $this->bucket->downloadToStream('id', $destination);
     }
 
-    public static function provideInvalidStreamValues()
+    public static function provideInvalidStreamValues(): array
     {
         return self::wrapValuesForDataProvider(self::getInvalidStreamValues());
     }
@@ -1029,7 +1029,7 @@ private function assertIndexNotExists(string $collectionName, string $indexName)
     /**
      * Return a list of invalid stream values.
      */
-    private function getInvalidStreamValues(): array
+    private static function getInvalidStreamValues(): array
     {
         return [null, 123, 'foo', [], hash_init('md5')];
     }