Allow triggering :post_convert events atomically

[ashmaroli]

• This is a 🐛 bug fix.
• I've added tests.

Summary

Jekyll::Document instances allows triggering all registered hook events for owner :documents as well as an individual collection:

jekyll/lib/jekyll/document.rb

Lines 390 to 393 in 11ff8aa

	def trigger_hooks(hook_name, *args)
	Jekyll::Hooks.trigger collection.label.to_sym, hook_name, self, *args if collection
	Jekyll::Hooks.trigger :documents, hook_name, self, *args
	end

But this support was not extended for the post_convert hook event on master (hook event to be introduced in v4.2).

This pull request fixes the above discrepancy.

[DirtyF]

@jekyllbot: merge +fix