100-continue handled incorrectly

[dbrumley]

Describe the bug

From RFC: https://tools.ietf.org/html/rfc7231#section-5.1.1
Requirement for clients:

• A client MUST NOT generate a 100-continue expectation in a request that does not include a message body.

The current code always returns true, even with an empty message body.

Expected Behavior

Change to self._expect_header_set = headers.get('Content-Length') != '0'

Current Behavior

botocore will incorrectly generate a 100-continuation.

Reproduction Steps

Had this bug with minio. I don't have a repro local, but did cite the RFC.

Possible Solution

I'll submit a PR. It's a simple change.

Additional Information/Context

No response

SDK version used

develop

Environment details (OS name and version, etc.)

macos, python 3.9

[SamRemis]

Thank you for bringing this to our attention

We have had this issue reported a number of times; one of the boto maintainers gave an in depth explanation of the complexity of the issue here. We currently have an open PR for an experimental opt in header to solve this issue.

[github-actions]

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