fix: fix reports/reasons endpoint output

[bolinocroustibat]

Closes https://github.com/datagouv/data.gouv.fr/issues/1391

[ThibaudDauce]

You can also add the link to this PR to the changelog line about reports backend :-)

[bolinocroustibat]

Could you update this line of documentation?

    @ns.response(200, "dictionnary of available reasons associated with their labels", fields.Raw)

Otherwise, good for me!

Well spotted, done

[bolinocroustibat]

Perfect for udata-front :)

Indeed, done