Implement fixes, tests for #282 using helmetKey

[adewey]

This is a re-implementation of #282 which has been inactive for some time without response. Below is the original PR comment. The code was moved to the correct location and test written to verify.

This allows more fine-grained control over which tags are duplicated vs
overwritten.

• Tags of the same type with the same value in "helmetKey" will be overwritten, even if the would normally be duplicated.
• Tags of the same type with different values in "helmetKey" will be duplicated, even if they would normally be overwritten.

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[codecov]

Codecov Report

Merging #455 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #455      +/-   ##
==========================================
+ Coverage    97.6%   97.62%   +0.02%     
==========================================
  Files           3        3              
  Lines         292      295       +3     
==========================================
+ Hits          285      288       +3     
  Misses          7        7

Impacted Files	Coverage Δ
src/HelmetConstants.js	100% <ø> (ø)	⬆️
src/HelmetUtils.js	96.91% <100%> (+0.04%)	⬆️

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[FryDay]

👍

[Junhui-AppDirect]

I'm wondering if there is a timeline about when the react helmet key would be released and could be used?