Explicitly set user-agent in ffprobe pre-flight HEAD check

[calzoneman]

It appears that node.js' https module does not set any user-agent by default, as evidenced by this nginx log of my test file:

x.x.x.x - - [11/May/2020:00:35:18 +0000] "HEAD /bin93tn1.webm HTTP/1.1" 200 0 "-" "-"

This may break certain websites. The ffprobe pre-flight HEAD check should explicitly set the user-agent to some reasonable value.

[calzoneman]

Fixed