fix: fix bugs when recursively appending middleware

[AllanZhengYP]

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[aws-sdk-js-automation]

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Codecov Report

Merging #476 into smithy-codegen will decrease coverage by 0.06%.
The diff coverage is 91.66%.

@@                Coverage Diff                 @@
##           smithy-codegen     #476      +/-   ##
==================================================
- Coverage            94.6%   94.53%   -0.07%     
==================================================
  Files                  68       68              
  Lines                1280     1282       +2     
  Branches              254      255       +1     
==================================================
+ Hits                 1211     1212       +1     
- Misses                 69       70       +1

Impacted Files	Coverage Δ
packages/middleware-stack/src/MiddlewareStack.ts	88.61% <91.66%> (-0.39%)	⬇️

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[AllanZhengYP]

Failed code cov, but it's just 1 line missing and it's hard to cover.

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