PGN format compliance

[ptitfred]

According to https://www.chessclub.com/user/help/PGN-spec, the 7 first (mandatory) headers are expected to be :

1. Event (the name of the tournament or match event)
2. Site (the location of the event)
3. Date (the starting date of the game)
4. Round (the playing round ordinal of the game)
5. White (the player of the white pieces)
6. Black (the player of the black pieces)
7. Result (the result of the game)

Round is missing, for instance: http://fr.lichess.org/game/export/FGI5RzDc.pgn

[Event "Casual game"]
[Site "http://lichess.org/FGI5RzDc"]
[Date "2016.05.23"]
[White "ptitfred"]
[Black "tOk_W"]
[Result "0-1"]
[WhiteElo "1422"]
[BlackElo "1500"]
[PlyCount "38"]
[Variant "Standard"]
[TimeControl "240+0"]
[ECO "D35"]
[Opening "Queen's Gambit Declined: Normal Defense"]
[Termination "Normal"]
[Annotator "lichess.org"]

1. d4 d5 2. c4 e6 3. Nc3 Nf6 { D35 Queen's Gambit Declined: Normal Defense } 4. a3 Bd6 5. Nf3 O-O 6. Ne5 Nbd7 7. Bf4 Bxe5 8. dxe5 Ne8 9. e4 dxc4 10. Bxc4 Qh4 11. Qf3 c6 12. a4 Nb6 13. g3 Qe7 14. Qg4 Nxc4 15. Bh6 Qb4 16. Qxg7+ Nxg7 17. Bxg7 Kxg7 18. O-O Qxb2 19. Ne2 Qxe2 { White resigns } 0-1

According to the spec, we could simply put [Round "-"]

The Round tag value gives the playing round for the game. In a match
competition, this value is the number of the game played. If the use of a
round number is inappropriate, then the field should be a single hyphen
character. If the round is unknown, a single question mark should appear as
the tag value.

[niklasf]

This was recently fixed: lichess-org/lila#2989, lichess-org/lila@48c39e6