Attempt to filter expand_fields before stringifying them

[shaunagm]

Two years ago, filter logic was added to the get_person VAN method to remove fields that didn't exist on particular databases. Unfortunately, it was added after the logic which turns expand_fields from an array into a string, leading to weirdness like this:

This in turn leads to 404 errors. Has get_person just always returned 404 errors for the last two years? Is no one using the expand_fields parameter? Have people been silently working around this? I am deeply curious.

Note: this PR also makes it so the print statement displays vanid instead of none when a vanid is passed in.

[shaunagm]

Note: this may be related to #673

[SorenSpicknall]

Thanks for catching this. I'm also very curious about what people were doing up to this point, especially since it's such a heavily used connector.