ReadPreference.NEAREST can route operations to arbiters in PyMongo 3.0

Setup is 3 node replica-set consisting of
1 Primary, 1 Secondary, 1 Arbiter.

Setting `w=2` and `read_preference=pymongo.ReadPreference.NEAREST` results in getting wrong results to queries occasionally.

See attached example program. Output from the program on my system is:

python pymongo_bug.py
found 1 docs
found 0 docs WHICH IS WRONG
found 0 docs WHICH IS WRONG
found 4 docs
found 0 docs WHICH IS WRONG
found 6 docs
found 7 docs
...

Setting read_preference to Primary or Secondary solves the problem.