[fix-signed-headers] aws signatures v4: don't actually check Content-Type is signed

This page of the AWS docs indicate that Content-Type should be part of
the CanonicalHeaders (and therefore SignedHeaders) strings in signature
calculation:

https://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-header-based-auth.html

However, testing with Minio Client revealed that it did not sign the
Content-Type header, and therefore we broke CI by expecting it to be
signed. With this commit, we don't mandate Content-Type to be signed
anymore, for better compatibility with the ecosystem. Testing against
the official behavior of S3 on AWS has not been done.

script/test-smoke.sh

@@ -81,11 +81,9 @@ if [ -z "$SKIP_AWS" ]; then
     echo "Invalid multipart upload"
     exit 1
   fi
+  aws s3api delete-object --bucket eprouvette --key upload
 fi
 
-echo "OK!!"
-exit 0
-
 # S3CMD
 if [ -z "$SKIP_S3CMD" ]; then
   echo "🛠️ Testing with s3cmd"