Preserve custom content-encoding value when setting `aws-chunked` (#4099)

## Motivation and Context
aws-sdk-rust#1281

## Description
When setting `aws-chunked` to the `Content-Encoding` header for
streaming APIs, we overwrite an existing custom value, if any provided
by the customer. The expected behavior is to support multiple values as
described in [the
documentation](https://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-streaming.html).

## Testing
Updated existing unit test to account for a custom `Content-Encoding`
header value.

## Checklist
- [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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