We use it like this:
expect(message.created_at).to eq_time(created_at)
The problem is that if one of the values or both of them are `nil` the matcher fails
with this error:
NoMethodError: undefined method `-' for nil:NilClass
This commit adds support for `nil` values. If both time values are `nil` they are equal,
if only one value is `nil` they aren't.
c5c043d9e2