Support fields in Live Query subscriptions

[TimNZ]

Use field keys specified via query.select() as fields value for subscribe.

Default fields of [className,objectId, createdAt, updatedAt, ACL] are always sent by server, which means the LQ subscribe message description for query.fields is inaccurate.

https://github.com/parse-community/parse-server/wiki/Parse-LiveQuery-Protocol-Specification#subscribe-message

I don't think ACL should be in the default fields but I don't know yet if it's used in the client somehow. Looking at that is another task.

[codecov]

Codecov Report

Merging #518 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #518      +/-   ##
==========================================
+ Coverage   84.57%   84.58%   +0.01%     
==========================================
  Files          48       48              
  Lines        3947     3951       +4     
  Branches      895      897       +2     
==========================================
+ Hits         3338     3342       +4     
  Misses        609      609

Impacted Files	Coverage Δ
...arse-community/Parse-SDK-JS/src/LiveQueryClient.js	87.27% <0%> (+0.31%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b27ff60...092b089. Read the comment docs.

[flovilmart]

Perfect @TimNZ ! Just checked and you're 💯 parse-server supports the fields option from the get-go!

[montymxb]

Thanks a ton @TimNZ !

[TimNZ]

I'm not a coder wizz but always interested in following best/popular practice

Would you have written:

let fields = queryJSON.keys ? queryJSON.keys.split(',') : undefined;

as

let fields = queryJSON.keys && queryJSON.keys.split(',')