Return HTTP 204 instead of GIF for JavaScript tracking.

#6577 Added a new url parameter `send_image=0` to disable sending an image and instead response with a 204 code. Added a couple of tests. Not sure if I forgot anything. Might be worth using this parameter in iOS & Android SDK? Shall we create issues for this?

We'll have to update doc after merging it.
why is the 1.0 protocol used instead of 1.1 ?