Do not automatically include the Content-MD5 HTTP header

[omus]

Fixes #695.

[mattBrzezinski]

I guess to include this functionality users will need to specify the header themselves? Does the removal of this have other implications?

[omus]

If someone is using sig_aws4! directly they could include the header by adding it into the request. Other typical AWS.jl use cases will no exclude the header which as shown in #695 isn't required by any of the API calls and additionally we always include the x-amz-content-sha256 header.

[omus]

Integration tests caught an issue:

<Error><Code>InvalidRequest</Code><Message>Missing required header for this request: Content-MD5 OR x-amz-checksum-*</Message><RequestId>03V7TDDC5SETNQES</RequestId><HostId>jnbR5hz0lH+e7x6pAjodJZiBdy1ek0J0dFOAYqgR+GQrH/5XJXq5UY2i/N0NHF0TRSdza1Gg2zo=</HostId></Error>

I think what happened is I did my analysis on newer service definitions and older ones require Content-MD5 or x-amz-checksum-* like the error states.

[omus]

I think I'll push this into the v2 release as this should be more straight forward to apply. Possibly I still could run into needing to add x-amz-checksum-* which may not be marked as required as there are multiple algorithms to choose from: https://docs.aws.amazon.com/AmazonS3/latest/userguide/checking-object-integrity.html