Add ability to test transfer time

src/Handler/MockHandler.php

@@ -182,7 +182,8 @@ private function invokeStats(
         $reason = null
     ) {
         if (isset($options['on_stats'])) {
-            $stats = new TransferStats($request, $response, 0, $reason);
+            $transferTime = isset($options['transfer_time']) ? $options['transfer_time'] : 0;
+            $stats = new TransferStats($request, $response, $transferTime, $reason);
             call_user_func($options['on_stats'], $stats);
         }
     }

tests/Handler/MockHandlerTest.php

@@ -221,4 +221,18 @@ public function testInvokesOnStatsFunctionForError()
         $this->assertNull($stats->getResponse());
         $this->assertSame($request, $stats->getRequest());
     }
+
+    public function testTransferTime()
+    {
+        $e = new \Exception('a');
+        $c = null;
+        $mock = new MockHandler([$e], null, function ($v) use (&$c) { $c = $v; });
+        $request = new Request('GET', 'http://example.com');
+        $stats = null;
+        $onStats = function (TransferStats $s) use (&$stats) {
+            $stats = $s;
+        };
+        $mock($request, ['on_stats' => $onStats, 'transfer_time' => 0.4])->wait(false);
+        $this->assertEquals(0.4,$stats->getTransferTime());
+    }
 }