zstd: use SpeedBestCompression for level >= 10 (#410)

Author: atetubou

zstd/encoder_options.go

@@ -189,7 +189,7 @@ func EncoderLevelFromZstd(level int) EncoderLevel {
 case level >= 6 && level < 10:
 return SpeedBetterCompression
 case level >= 10:
-return SpeedBetterCompression
+return SpeedBestCompression
 }
 return SpeedDefault
 }