Fix s3 `checksum-mode` header in presigned requests (#4000)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->

Fixes a bug reported in
https://github.com/awslabs/aws-sdk-rust/issues/1253 with flexible
checksum headers sneaking into presigned requests.

## Description
<!--- Describe your changes in detail -->
Short circuit the closure that mutates the `checksum-mode` header when
we detect that we are making a presigned request.

## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->
Update our presigned canary tests to do both PUT and GET and have extra
headers included that aren't in the query string that will expose this
issue if there is a regression.


## Checklist
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rather than leaving it unchecked -->
- [x] For changes to the AWS SDK, generated SDK code, or SDK runtime
crates, I have created a changelog entry Markdown file in the
`.changelog` directory, specifying "aws-sdk-rust" in the `applies_to`
key.

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