X-Tracy-Ajax is added to a cross-origin request

[jiripudil]

Version: >= 2.4.2

Bug Description

The change in #211 doesn't take into account the situation when the scheme-less URL (//anything.example.com) points to a different origin. In such case the X-Tracy-Ajax header is added to a cross-origin request and the preflight request fails because it doesn't list the tracy header in Access-Control-Allow-Headers.

[jiripudil]

Wow, that was quick, thank you!