Fix ObjectID and other type equality assertions for HHVM

HHVM does not support comparing ObjectIDs with the equality operator (i.e. `==`). For documents, which may contain arbitrary BSON types, converting to JSON is the easiest solution. The documents will then be compared as extended JSON and produce a readable diff on error.

tests/FunctionalTestCase.php

@@ -40,11 +40,18 @@ abstract class FunctionalTestCase extends TestCase
         $this->assertEquals(1, $document['ok']);
     }
 
+    protected function assertSameObjectID($expectedObjectID, $actualObjectID)
+    {
+        $this->assertInstanceOf('MongoDB\BSON\ObjectID', $expectedObjectID);
+        $this->assertInstanceOf('MongoDB\BSON\ObjectID', $actualObjectID);
+        $this->assertEquals((string) $expectedObjectID, (string) $actualObjectID);
+    }
+
     protected function assertSameDocument($expectedDocument, $actualDocument)
     {
         $this->assertEquals(
-            $this->normalizeBSON($expectedDocument),
-            $this->normalizeBSON($actualDocument)
+            \MongoDB\BSON\toJSON(\MongoDB\BSON\fromPHP($this->normalizeBSON($expectedDocument))),
+            \MongoDB\BSON\toJSON(\MongoDB\BSON\fromPHP($this->normalizeBSON($actualDocument)))
         );
     }
 
@@ -59,7 +66,7 @@ abstract class FunctionalTestCase extends TestCase
         }
 
         $normalizeRootDocuments = function($document) {
-            return $this->normalizeBSON($document);
+            return \MongoDB\BSON\toJSON(\MongoDB\BSON\fromPHP($this->normalizeBSON($document)));
         };
 
         $this->assertEquals(

tests/GridFS/BucketFunctionalTest.php

@@ -331,7 +331,7 @@ class BucketFunctionalTest extends FunctionalTestCase
 
         $fileDocument = $this->bucket->getFileDocumentForStream($stream);
 
-        $this->assertEquals($id, $fileDocument->_id);
+        $this->assertSameObjectID($id, $fileDocument->_id);
         $this->assertSame('filename', $fileDocument->filename);
         $this->assertSame(6, $fileDocument->length);
         $this->assertSameDocument($metadata, $fileDocument->metadata);
@@ -363,7 +363,7 @@ class BucketFunctionalTest extends FunctionalTestCase
         $id = $this->bucket->uploadFromStream('filename', $this->createStream('foobar'));
         $stream = $this->bucket->openDownloadStream($id);
 
-        $this->assertEquals($id, $this->bucket->getFileIdForStream($stream));
+        $this->assertSameObjectID($id, $this->bucket->getFileIdForStream($stream));
     }
 
     public function testGetFileIdForStreamWithWritableStream()