Fix Sigv4 signing bug for endpoints with default ports (#4006)

## Motivation and Context
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Bug from https://github.com/awslabs/aws-sdk-rust/issues/1244

## Description
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Fix bug in Sigv4 signing that, when an endpoint contained a default port
(80 for HTTP, 443 for HTTPS), would sign the request with that port in
the `HOST` header even though Hyper excludes default ports from the
`HOST` header.

## Testing
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to -->
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Added new unit test to confirm that default ports are appropriately
stripped from HTTP and HTTPS requests.

## 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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