Duplicated creators when registering new workflow versions

[fbacall]

In the workflow metadata extractors we use assets_creators_attributes to add creators, which will only ever add new creators, it won't remove or updating existing ones:

seek/lib/seek/workflow_extractors/nextflow.rb

Lines 11 to 22 in c59d3f8

	mani = manifest
	metadata[:title] = mani['name'] if mani.has_key?('name')
	metadata[:description] = mani['description'] if mani.has_key?('description')
	if mani['author'].present?
	mani['author'].split(',').each_with_index do |author_name, i|
	author = extract_author(author_name)
	unless author.blank?
	metadata[:assets_creators_attributes] ||= {}
	metadata[:assets_creators_attributes][i.to_s] = author.merge(pos: i)
	end
	end
	end

So every time a new version of a workflow is added (when scraping, for example), it will keep adding to the list.