Demonstrate edge case where replacement doc encodes as pipeline array

Author: jmikola

tests/Operation/UpdateFunctionalTest.php

@@ -52,6 +52,7 @@ function (array $event) use ($expectedFilter): void {
      * @dataProvider provideReplacementDocuments
      * @dataProvider provideUpdateDocuments
      * @dataProvider provideUpdatePipelines
+     * @dataProvider provideReplacementDocumentLikePipeline
      */
     public function testUpdateDocuments($update, $expectedUpdate): void
     {
@@ -76,6 +77,18 @@ function (array $event) use ($expectedUpdate): void {
         );
     }
 
+    public function provideReplacementDocumentLikePipeline(): array
+    {
+        /* libmongoc encodes this replacement document as a BSON array because
+         * it resembles an update pipeline (see: CDRIVER-4658). */
+        return [
+            'replacement_like_pipeline' => [
+                (object) ['0' => ['$set' => ['x' => 1]]],
+                [(object) ['$set' => (object) ['x' => 1]]],
+            ],
+        ];
+    }
+
     public function testSessionOption(): void
     {
         (new CommandObserver())->observe(