Fix CloudWatch TestChecksumInvoke and TestCompressionCall unit tests

[muhammad-othman]

Description

• TestChecksumInvoke sometime fails when additional headers are added during the marshalling, this change helps preventing that by only checking for checksum headers.
• TestCompressionCall tests were also failing in some cases when the request body is added to the .Content.

Motivation and Context

Fixing the unit tests.

Testing

• Ran the unit tests locally with different protocols.

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed

License

• I confirm that this pull request can be released under the Apache 2 license