Start iteration with rewind() in ChangeStream::key() test

Previously, iteration started with next(), so the first element's key was 1. Iteration should being with rewind() and the initial key should be zero.

With this change, we're still testing that the key is only incremented when next() successfully increments.

tests/Operation/WatchFunctionalTest.php

@@ -180,13 +180,14 @@ class WatchFunctionalTest extends FunctionalTestCase
         $operation = new Watch($this->manager, $this->getDatabaseName(), $this->getCollectionName(), [], ['maxAwaitTimeMS' => 100]);
         $changeStream = $operation->execute($this->getPrimaryServer());
 
+        $this->assertFalse($changeStream->valid());
         $this->assertNull($changeStream->key());
 
         $this->insertDocument(['_id' => 1, 'x' => 'foo']);
 
-        $changeStream->next();
+        $changeStream->rewind();
         $this->assertTrue($changeStream->valid());
-        $this->assertSame(1, $changeStream->key());
+        $this->assertSame(0, $changeStream->key());
 
         $changeStream->next();
         $this->assertFalse($changeStream->valid());
@@ -206,7 +207,7 @@ class WatchFunctionalTest extends FunctionalTestCase
 
         $changeStream->next();
         $this->assertTrue($changeStream->valid());
-        $this->assertSame(2, $changeStream->key());
+        $this->assertSame(1, $changeStream->key());
     }
 
     public function testNonEmptyPipeline()