Don't remove content-type from signed headers

[adam-fowler]

This is from S3 sigV4 docs

The CanonicalHeaders list must include the following:

• HTTP host header.
• If the Content-Type header is present in the request, you must add it to the CanonicalHeaders list.
• Any x-amz-* headers that you plan to include in your request must also be added. For example, if you are using temporary security credentials, you need to include x-amz-security-token in your request. You must add this header in the list of CanonicalHeaders.

[codecov[bot]]

Codecov Report

Merging #561 (98cd864) into main (dba58d0) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #561      +/-   ##
==========================================
+ Coverage   77.12%   77.14%   +0.01%     
==========================================
  Files          87       87              
  Lines        8551     8557       +6     
==========================================
+ Hits         6595     6601       +6     
  Misses       1956     1956              

Impacted Files	Coverage Δ
Sources/SotoSignerV4/signer.swift	97.34% <100.00%> (+0.05%)	:arrow_up:

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