Fix bug: media_sequence can be zero

[mojtaba-arvin]

The zero media sequence was ignored in the media playlist. (The zero media sequence as default is important for the proper functioning of the players.)

[davemevans]

This breaks a bunch of the existing tests, and has no accompanying test.

Also, the spec is explicit that if the tag is not in the playlist the value shall be considered to be 0 so it's interesting that this breaks players?

[mojtaba-arvin]

This breaks a bunch of the existing tests, and has no accompanying test.

Also, the spec is explicit that if the tag is not in the playlist the value shall be considered to be 0 so it's interesting that this breaks players?

 The Seek bar of the player will break and the player will show only ten seconds (the length of a segment) of a one-minute HLS. 
 

 https://github.com/globocom/m3u8/pull/259#issuecomment-910399500