Do not allow to protect default fields

[BufferUnderflower]

No description provided.

[davimacedo]

I'm not sure. I think that protecting the default fields it will make the SDKs to fail. So I'm wondering if it is really a good idea to implement a feature in the server side knowing that the SDKs will fail if used. @dplewis or @acinader may have a different vision about this.

[codecov]

Codecov Report

Merging #6439 into master will decrease coverage by 0.56%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6439      +/-   ##
==========================================
- Coverage   93.96%   93.39%   -0.57%     
==========================================
  Files         169      169              
  Lines       11811    11811              
==========================================
- Hits        11098    11031      -67     
- Misses        713      780      +67

Impacted Files	Coverage Δ
src/Controllers/SchemaController.js	96.91% <100%> (ø)	⬆️
...dapters/Cache/RedisCacheAdapter/KeyPromiseQueue.js	0% <0%> (-95.46%)	⬇️
src/Adapters/Cache/RedisCacheAdapter/index.js	14.28% <0%> (-83.68%)	⬇️
src/Routers/PushRouter.js	93.1% <0%> (-3.45%)	⬇️
src/Adapters/Storage/Mongo/MongoStorageAdapter.js	92.79% <0%> (-0.68%)	⬇️
src/RestWrite.js	93.64% <0%> (-0.17%)	⬇️

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[BufferUnderflower]

I'm not sure. I think that protecting the default fields it will make the SDKs to fail. So I'm wondering if it is really a good idea to implement a feature in the server side knowing that the SDKs will fail if used. @dplewis or @acinader may have a different vision about this.

Reasonable, I was working on dashboard and noticed 'field does not exist' error when tried to protect some of default fields.

So another solution would be to just fine tune that message and agree to disallow protecting default fields.

[davimacedo]

Looks good to me. LGTM!