Optimistically set ACLs from JSON blobs

[andrewimm]

Fixes #23

[codecov-io]

Current coverage is 84.00%

Merging #30 into master will increase coverage by +0.01% as of 826213c

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  Files           37     37       
  Stmts         2743   2745     +2
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+ Hit           2304   2306     +2
  Partial          0      0       
  Missed         439    439       

Review entire Coverage Diff as of 826213c

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[wangmengyan95]

Code LGTM, but why this cause other fields to become undefined?

[andrewimm]

Validation is performed on every set(), and the default validation is to ensure that values in the ACL field are instanceof ParseACL. Cloud code performs an inner set() on beforeSave, so that the fields that were updated appear dirty within cloud code, but it was receiving a JSON-form ACL and failing that set.