[Bug] WebVTT does not follow spec as there is no blank line

[rbozan]

Describe the bug
The WebVTT format when downloading subtitles does not follow the WebVTT spec as there should be a blank line after the first line which is WebVTT.

One place where this is described is here:
https://developer.mozilla.org/en-US/docs/Web/API/WebVTT_API#webvtt_body

Steps to Reproduce

1. Go to a video, for example https://iv.melmac.space/watch?v=t0mQqqNuOg4
2. Click on 'Download subtitle' on the left
3. Open the file
4. Notice how there is no blank line after WebVTT

EDIT: my fault :/