Fix PresignedUrl issue with case convertion of header names

[jderusse]

an alternative to #2044
that should fix #2043 without breaking #2035

Could you test this patch @root-aza and @ryanmab ?

[ryanmab]

Hey @jderusse,

Thanks for the work on this PR!

I've just pulled this version in locally on my side, but no change I'm afraid - the signed PUT requests are still rejected by S3 with the error response of:

<Error>
	<Code>SignatureDoesNotMatch</Code>
	<Message>The request signature we calculated does not match the signature you provided. Check your key and signing method.</Message>
        // .....
</Error>

For what it's worth, I wanted to rule out that I'd not tested the patch correctly, so also tried pinning async-aws/core to 1.28.0 and then applying the changes in the core and s3 packages manually file by file.

Before applying the changes, the signed PUT requests worked against S3 (i.e. just plain 1.28.0 of async-aws/core), but after applying this patch to each file manually, the signed URLs to began failing - so I believe I'm exercising the Signer correctly.

I realise thats not a while lot to go on though, so let me know if theres anything outside of the original issue or my follow-up PR which might help you track it down.

Ryan