allow to unset a previously set RANGE value

... and add the unset_range_test.py unit test exercising it

Reported by: Zdeněk Pavlas
Bug: https://bugzilla.redhat.com/928370
allow to use setopt(..., None) as unsetopt()

... and extend the unset_range_test.py unit test to exercise it

Reported by: Zdeněk Pavlas
Bug: https://bugzilla.redhat.com/928370
Merge pull request #29 from kdudka/bz928370

[RFE] support setopt(pycurl.RANGE, None)