MIM: Incorrect positional $ behavior sometimes

[brondsem]

While working on the fix of #25, I found that sometimes the wrong behavior occurs. On branch mim_dot_dollar_wrong this is illustrated by test_inc_dotted_dollar_middle2 with a $ in the middle of dotted field notation, and selecting for the 2nd subdoc in the list:

Ming/ming/tests/test_mim.py

Lines 213 to 217 in 256f2b1

	def test_inc_dotted_dollar_middle2(self):
	# match on g=2 and $inc by 10
	self.coll.update({'b.f.g': 2}, { '$inc': { 'b.f.$.g': 10 } })
	obj = self.coll.find_one({}, { '_id': 0, 'b.f': 1 })
	self.assertEqual(obj, { 'b': { 'f': [ { 'g': 1 }, { 'g': 12 } ] }})

AssertionError: {'b': {'f': [{'g': 11}, {'g': 2}]}} != {'b': {'f': [{'g': 1}, {'g': 12}]}}
- {'b': {'f': [{'g': 11}, {'g': 2}]}}
?                    -

+ {'b': {'f': [{'g': 1}, {'g': 12}]}}
?

That branch has expanded tests where the $ is at the end of dotted field notation, and they do all pass.

I haven't looked into this any further, seems kinda tricky. And not actually a blocking problem for me right now.