Fix the not exist case in $toInt and $toDecimal

[lxy1992]

It's the fix for the bug I mentioned in the last PR.

First, I thought it was a serious bug in _Parse function, so I haven't fix it in the last PR. Now, I believe it only affect the operators in the $addField operator. So, I fix them in this PR with UTs.

The case is, when we try to add a field from a field which is not exist, Mongo will return the new field with the value of None.

Now, the $toInt and $toDecimal operators will return the original query now. So I return None when the KeyError raised in these operators.

If you're convenient, please review and merge it.

Cheers!

[codecov]

Codecov Report

Merging #612 into develop will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #612   +/-   ##
========================================
  Coverage    94.74%   94.75%           
========================================
  Files           16       16           
  Lines         3217     3223    +6     
========================================
+ Hits          3048     3054    +6     
  Misses         169      169           

Impacted Files	Coverage Δ
mongomock/aggregate.py	97.03% <100.00%> (+0.02%)	⬆️

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[lxy1992]

I am working on another PR about $sum. I may finish it at this weekend. Looking forward your code review, thanks!

[pcorpet]

Perfect, thanks.

Feel free to send a PR to add yourself as a contributor to this project in the main page.

[lxy1992]

OK, I will add this info in the next PR. Thanks!