fix(associations): ensure correct schema on all generated attributes

[sushantdhiman]

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• Does npm run test or npm run test-DIALECT pass with this change (including linting)?
• Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
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Description of change

Fixes an general issue with association, improper merging of newly generated attributes was overriding schema based toString fn with Object.isString

Closes #10125

[codecov]

Codecov Report

Merging #12258 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #12258      +/-   ##
==========================================
- Coverage   96.34%   96.33%   -0.01%     
==========================================
  Files          95       95              
  Lines        9126     9125       -1     
==========================================
- Hits         8792     8791       -1     
  Misses        334      334              

Impacted Files	Coverage Δ
lib/dialects/mssql/query-generator.js	95.69% <ø> (ø)
lib/dialects/mysql/query-generator.js	97.61% <ø> (ø)
lib/associations/helpers.js	100.00% <100.00%> (ø)
lib/utils.js	98.31% <100.00%> (+0.01%)	⬆️

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