Handle ACL on save

[wangmengyan95]

It seems we do not handle ACL on save. Normally for a save/update, the server just return createAt, objectId and updateAt, so it is fine. However, if the developers enable beforeSave for this class, the server will return the full object. In this situation, if we do not handle ACL, it will make object.getACL() return null.
I follow the way we handle ACL in _finishFetch to handle ACL. Not sure it is the best way to do that.

[wangmengyan95]

@hallucinogen not sure andrew has time to review, so could you also take a look?
cc @peterdotjs .

[codecov-io]

Current coverage is 78.54%

Merging #123 into master will increase coverage by +0.16% as of 928fc44

@@            master    #123   diff @@
======================================
  Files           37      37       
  Stmts         2822    2824     +2
  Branches       677     678     +1
  Methods          0       0       
======================================
+ Hit           2212    2218     +6
+ Partial        194     193     -1
+ Missed         416     413     -3

Review entire Coverage Diff as of 928fc44

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[andrewimm]

lgtm, ship it