fix: add Content-Type and Content-Length to unsignable headers

[trivikr]

Issue #, if available:
Fixes: #1000

Description of changes:
add Content-Type and Content-Length to unsignable headers

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[aws-sdk-js-automation]

AWS CodeBuild CI Report

• CodeBuild project: sdk-staging-test
• Commit ID: c53b729
• Result: FAILED
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[codecov-io]

Codecov Report

Merging #1014 into smithy-codegen will not change coverage.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           smithy-codegen    #1014   +/-   ##
===============================================
  Coverage           91.36%   91.36%           
===============================================
  Files                 144      144           
  Lines                2836     2836           
  Branches              501      501           
===============================================
  Hits                 2591     2591           
  Misses                245      245

Impacted Files	Coverage Δ
packages/signature-v4/src/constants.ts	100% <ø> (ø)	⬆️

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[aws-sdk-js-automation]

AWS CodeBuild CI Report

• CodeBuild project: sdk-staging-test
• Commit ID: 5631d87
• Result: FAILED
• Build Logs (available for 30 days)

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[trivikr]

Superseded by #1017

[lock]

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