Should Timecop.travel(nil) raise an exception like Timecop.travel("")?

[mottalrd]

Hello, I find this behavior a bit odd

irb(main):001:0> require 'timecop'
=> true
irb(main):002:0> Timecop.travel(nil) do end
=> nil
irb(main):003:0> Timecop.travel("") do end
ArgumentError: no time information in ""
	from /Users/motta/.rbenv/versions/2.3.1/lib/ruby/2.3.0/time.rb:254:in `make_time'

shouldn't Timecop.travel(nil) behave in the same way as Timecop.travel("")?
What's the rationale for this choice?

I got bitten by this recently :)

[andyjdavis]

I have raised a PR containing test to confirm the empty string behaviour. #369

I believe passing in nil is valid as there is a default. Now?