Fixes bug when querying equalTo on objectId and relation

[flovilmart]

• Adds $eq operator in transform
• Makes $eq operator on objectId when adding $in operator

Fixes #801

[codecov-io]

Current coverage is 91.72%

Merging #887 into master will increase coverage by +0.02% as of 275b3ee

@@            master    #887   diff @@
======================================
  Files           71      71       
  Stmts         4110    4110       
  Branches       849     849       
  Methods          0       0       
======================================
+ Hit           3769    3770     +1
  Partial         10      10       
+ Missed         331     330     -1

Review entire Coverage Diff as of 275b3ee

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