public function testExtensionsAlwaysOverwritten()
 {
     $ext1 = ['defdata' => 'fghi'];
     $ext2 = ['data1' => '1234', 'data2' => 'abcd'];
     $problem = new HTTPProblem(400, 'msg', $ext1);
     $this->assertSame($ext1, $problem->extensions());
     $problem->withExtensions($ext2);
     $this->assertSame($ext2, $problem->extensions());
 }