In attach_story_data_to_stories(), don't overwrite existing fields

[pypt]

Hey Hal!

I know you're superbusy with other stuff, but could you have a quick look?

In https://github.com/mediacloud/backend/issues/729 (and possibly https://github.com/mediacloud/backend/issues/725), it looks like MediaWords::DBI::Stories::attach_story_data_to_stories() used to reset $list_field key to an empty arrayref on every call, thus making this subroutine not work when it was called multiple times, e.g. when data was being "attached" in chunks:

https://github.com/mediacloud/backend/blob/12a0c0e896bb6979469248d43f4a7f1ec23c3d8e/apps/webapp-api/src/perl/MediaWords/Controller/Api/V2/StoriesBase.pm#L279-L302

I've now made the subroutine merge the input $story_data with whatever might already be in the parameter $stories under $list_field (if anything). There's also a test that verifies that the subroutine behaves as intended.

Does it look good to you?

[hroberts]

looks good to me. nice catch.