lavf/mux: Don't write "encoder" metadata tag when bitexact is requested

Since we don't write lavf's string when bitexact is requested, this will
prevent the tag from being copied from the source stream.
Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mux.c

@@ -339,6 +339,8 @@ static int init_muxer(AVFormatContext *s, AVDictionary **options)
     /* set muxer identification string */
     if (s->nb_streams && !(s->streams[0]->codec->flags & CODEC_FLAG_BITEXACT)) {
         av_dict_set(&s->metadata, "encoder", LIBAVFORMAT_IDENT, 0);
+    } else {
+        av_dict_set(&s->metadata, "encoder", NULL, 0);
     }
 
     if (options) {

tests/ref/lavf-fate/ogg_vp3

-adbe6c30bdfe934dc5ae397f4db2960d *./tests/data/lavf-fate/lavf.ogg
-417644 ./tests/data/lavf-fate/lavf.ogg
+4bd51dac3194fa88ae33767c25b4b1e6 *./tests/data/lavf-fate/lavf.ogg
+417621 ./tests/data/lavf-fate/lavf.ogg
 ./tests/data/lavf-fate/lavf.ogg CRC=0x037e3e79