Enable streaming upload in S3 : put_object

[oconnore]

There were two problems:

1. Request did not allow an IO
2. Signing did not permit an unsigned payload
   (https://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-header-based-auth.html)

[ararslan]

Apologies for the long silence here. Could you add some tests to ensure this works as intended?

[ararslan]

You can write this a bit more concisely like so:

Suggested change

	if !haskey(request.headers, "x-amz-content-sha256")
	content_hash = bytes2hex(digest(MD_SHA256, request.content))
	request.headers["x-amz-content-sha256"] = content_hash
	else
	content_hash = "UNSIGNED-PAYLOAD"
	end
	content_hash = get!(request.headers, "x-amz-content-sha256") do
	bytes2hex(digest(MD_SHA256, request.content))
	end

[ararslan]

This repository uses the Blue style guide, which puts closing parentheses on the following line in cases like this

Suggested change

	digest(MD_MD5, request.content))
	digest(MD_MD5, request.content)
	)

Supposedly automatic formatting suggestions are set up but that appears to not have occurred here.