Does not error when `date` is missing from YAML

[ethanwhite]

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I recently submitted a paper to JOSE. We checked compilation using the action and it complied successfully, but when it was compiled by whedon as part of the submission it failed to compile. The failure was due to a missing date value in the YAML front matter.

 /app/vendor/bundle/ruby/2.6.0/bundler/gems/whedon-c5c16aedb3d6/lib/whedon.rb:147:in `check_fields': Paper YAML header is missing expected fields: date (RuntimeError)
	from /app/vendor/bundle/ruby/2.6.0/bundler/gems/whedon-c5c16aedb3d6/lib/whedon.rb:89:in `initialize'
	from /app/vendor/bundle/ruby/2.6.0/bundler/gems/whedon-c5c16aedb3d6/lib/whedon/processor.rb:38:in `new'
	from /app/vendor/bundle/ruby/2.6.0/bundler/gems/whedon-c5c16aedb3d6/lib/whedon/processor.rb:38:in `set_paper'
	from /app/vendor/bundle/ruby/2.6.0/bundler/gems/whedon-c5c16aedb3d6/bin/whedon:58:in `prepare'
	from /app/vendor/bundle/ruby/2.6.0/gems/thor-0.20.3/lib/thor/command.rb:27:in `run'
	from /app/vendor/bundle/ruby/2.6.0/gems/thor-0.20.3/lib/thor/invocation.rb:126:in `invoke_command'
	from /app/vendor/bundle/ruby/2.6.0/gems/thor-0.20.3/lib/thor.rb:387:in `dispatch'
	from /app/vendor/bundle/ruby/2.6.0/gems/thor-0.20.3/lib/thor/base.rb:466:in `start'
	from /app/vendor/bundle/ruby/2.6.0/bundler/gems/whedon-c5c16aedb3d6/bin/whedon:131:in `<top (required)>'
	from /app/vendor/bundle/ruby/2.6.0/bin/whedon:23:in `load'
	from /app/vendor/bundle/ruby/2.6.0/bin/whedon:23:in `<main>'