remove Expect: 100-continue header when requesting an IMDSv2 access t…

[kamilJ96]

Relevant Issue (if applicable)

Fixes #1765 by explicitly setting the Expect header to be empty before making the request to retrieve the IMDSv2 token from /latest/api/token

Potentially fixes #1581 as well. Verified on my ec2-instance that the Expect 100 is removed from the request:

Sep  8 10:28:51:      Get IAM Role name
Sep  8 10:28:51: *   Trying 169.254.169.254:80...
Sep  8 10:28:51: * Connected to 169.254.169.254 (169.254.169.254) port 80 (#0)
Sep  8 10:28:51: > PUT /latest/api/token HTTP/1.1
Sep  8 10:28:51: > Host: 169.254.169.254
Sep  8 10:28:51: > User-Agent: s3fs/1.90 (commit hash 9d1552a; OpenSSL)
Sep  8 10:28:51: > Accept: */*
Sep  8 10:28:51: > Transfer-Encoding: chunked
Sep  8 10:28:51: > X-aws-ec2-metadata-token-ttl-seconds: 21600
Sep  8 10:28:51: >
Sep  8 10:28:51: * Signaling end of chunked upload via terminating chunk.
Sep  8 10:28:51: * Mark bundle as not supporting multiuse
Sep  8 10:28:51: * HTTP 1.0, assume close after body
Sep  8 10:28:51: < HTTP/1.0 200 OK
Sep  8 10:28:51: < Content-Length: 56
Sep  8 10:28:51: < Content-Type: text/plain
Sep  8 10:28:51: < Date: Wed, 08 Sep 2021 10:28:51 GMT
Sep  8 10:28:51: < X-Aws-Ec2-Metadata-Token-Ttl-Seconds: 21600
Sep  8 10:28:51: < Connection: close
Sep  8 10:28:51: < Server: EC2ws
Sep  8 10:28:51: <
Sep  8 10:28:51: * Closing connection 0
Sep  8 10:28:51:      HTTP response code 200
Sep  8 10:28:51:      Setting AWS IMDSv2 API token to *********************************

[gaul]

@nmeyerhans could you test and review this PR?

[nmeyerhans]

Changes look good to me. I can confirm that s3fs does not send a Expect: 100-continue header and it is able to retrieve an IMDS token on both Xen and Nitro instance types with the changes.

[gaul]

@kamilJ96 Thank you for your contribution!

@nmeyerhans Thank you for testing and reviewing!

[gaul]

Related StackOverflow question: https://stackoverflow.com/questions/69104247/s3fs-how-to-mount-s3-buckets-on-aws-ec2-instances-behind-proxy-using-imds-v2