DO NOT MERGE rangelib.py: Fix the bug in monotonic check.

RangeSet("2-10").monotonic gives a wrong result of "False". Fix the bug
and add more tests into test_rangelib.py.

Bug: 28760775
Change-Id: I04780571b45ecafd34040f405756b9745a9e21a5
(cherry picked from commit e8f756137c39fc3292014f2eab5f84c6e5135797)
1 file changed