V4 Signer incorrect behavior when ContentLength already set

[matelang]

Describe the bug

Hi,

I am trying to implement an AWS SDK v2 HTTP request Signer for the OpenSearch Go client library (traditionally support was only for AWS SDK V1 signer).

I've discovered that when having a request with a Content-Length already set, the AWS SDK V2 SignerV4 (package: github.com/aws/aws-sdk-go-v2/aws/signer/v4) will incorrectly re-append the content length to the canonical request as comma separated values.

Having the following test:

	b1, err := json.Marshal(map[string]interface{}{
		"description": "this is a test",
		"timestamp":   "2020-01-16T04:32:49",
	})
	if err != nil {
		log.Error().Err(err).Msg("json ctor error")
	}
	r1, err := http.NewRequest(http.MethodPut, "https://localhost:9200/testing/_doc/1?pretty",
		strings.NewReader(string(b1)))
	r1.Header.Set("Content-Type", "application/json")
	r1.Header.Set("Content-Length", fmt.Sprintf("%d", len(b1)))
	if err != nil {
		log.Error().Err(err).Msg("http ctor error")
	}

And calling the V1 signer and V2 signer on such a http.Request, will result in different signatures being generated, due to the canonical request being altered erroneously by the V2 implementation.

Expected canonical string:

2022/06/14 17:01:23 DEBUG Request Signature:
---[ CANONICAL STRING  ]-----------------------------
PUT
/testing/_doc/1
pretty=
content-length:66
content-type:application/json
host:localhost:9200
x-amz-date:20220614T140123Z

content-length;content-type;host;x-amz-date
a333350fb0ac11b4a9efea6cdf4dc98ddc1900c24a59530ec829e1189ab8172a

Actual canonical string (note content-length header):

---[ CANONICAL STRING  ]-----------------------------
PUT
/testing/_doc/1
pretty=
content-length:66,66
content-type:application/json
host:localhost:9200
x-amz-date:20220614T140309Z

content-length;content-type;host;x-amz-date
a333350fb0ac11b4a9efea6cdf4dc98ddc1900c24a59530ec829e1189ab8172a

Expected Behavior

Don't reappend the ContentLength as comma separated values.

Current Behavior

ContentLength header value gets appended as comma separated value.

Reproduction Steps

Please see issue description.

Possible Solution

#1729

Additional Information/Context

No response

AWS Go SDK V2 Module Versions Used

github.com/aws/aws-sdk-go-v2 v1.16.5
github.com/aws/aws-sdk-go-v2/config v1.15.10

Compiler and Version used

go version go1.18.3 darwin/arm64

Operating System and version

MacOs, M1 Pro

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