add support for mergeObjects operator

[elbehery95]

As per #642

The current implementation follows the operator as defined such that on all null an empty doc is returned, and the docs are merged using dict.update method

[pcorpet]

Thanks for your PR.

Please add multiple tests in test__mongomock to ensure there are no regressions in the future.

[elbehery95]

removed mergeobjects exception checking tests and added a single test__merge_objects under the aggregate tests

[pcorpet]

Thanks, almost there. Please fix the remaining styling issues
https://travis-ci.org/github/mongomock/mongomock/jobs/754673610

[codecov]

Codecov Report

Merging #689 (9f15d1b) into develop (fac6470) will decrease coverage by 0.10%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #689      +/-   ##
===========================================
- Coverage    95.45%   95.34%   -0.11%     
===========================================
  Files           19       19              
  Lines         3605     3612       +7     
===========================================
+ Hits          3441     3444       +3     
- Misses         164      168       +4     

Impacted Files	Coverage Δ
mongomock/aggregate.py	97.25% <100.00%> (-0.32%)	⬇️
mongomock/collection.py	92.40% <0.00%> (-0.08%)	⬇️

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[pcorpet]

Thanks @elbehery95 for the hard work!