FFmpegMetadataPP does not process `creator` field from info extractor

[blahgeek]

• I've verified and I assure that I'm running youtube-dl 2016.04.01

Before submitting an issue make sure you have:

• At least skimmed through README and most notably FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones

What is the purpose of your issue?

• Bug report (encountered problems with youtube-dl)

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In FFmpegMetadataPP:

        if info.get('artist') is not None:
            metadata['artist'] = info['artist']

Only artist field is used.

But in the documentation of InfoExtractor:

creator:        The main artist who created the video.

There's no artist field from any info extractor.

Shouldn't it be something like

        if info.get('creator') is not None:
            metadata['artist'] = info['creator']

?

Thanks!

[blahgeek]

Also, FFmpegMetadataPP uses album field as metadata, which is also not mentioned in the documentation of InfoExtractor