MIM: Avoid errors from _ensure_orig_key when positional $ is used

[brondsem]

There may well be a better way to do this, but this avoids an error from _ensure_orig_key when a positional $ is used in the middle of dotted notation. (This test passes with earlier versions of ming before _ensure_orig_key was added)

[coveralls]

Pull Request Test Coverage Report for Build 106

• 6 of 6 (100.0%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 88.627%

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Files with Coverage Reduction	New Missed Lines	%
ming/mim.py	1	75.89%

Totals
Change from base Build 99:	0.03%
Covered Lines:	5829
Relevant Lines:	6577

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💛 - Coveralls