Minor difference in date objects parsed

[RomainGehrig]

I found a difference between date object types returned when parsing nodes with only one line or multiple lines.

Eg:

from orgparse import loads
r0 = loads("""* Heading 1
* Heading 2
  body""")

print([c.scheduled for c in r0.children])  # prints [OrgDate(None), OrgDateScheduled(None)]

It's pretty minor and mostly cosmetic. I'm creating a PR to harmonize these a bit.