Motivation

We got a report with #12999 that sending a GetObjectAttributes request would fail depending on the order of the fields being passed.

While debugging and printing out low level HTTP request on the Java side, I realized that Java was sending the request with multiple headers for the same key, like some others SDK (Go in particular)

GET /test-key?attributes= HTTP/1.1
[...]
Amz-Sdk-Invocation-Id: 5cb65548-6947-4436-b731-ee6d618bc91c
Amz-Sdk-Request: attempt=1; max=3
Authorization: ---
X-Amz-Date: 20220316T000334Z
X-Amz-Object-Attributes: Checksum
X-Amz-Object-Attributes: ObjectParts

When receiving headers that way, our web server will return them as a list of values, accessible via Headers.getlist().

Because the parser was calling Headers.get(), it would only get the first value, and in the user sample above, would not even return the second requested attribute.

This PR adapts the parser to use getlist in the header shape type is list, and keep the old automatic splitting on , that is still required.

Changes

• add a parser unit test (thanks a lot for adding the headers override! This was so much easier to write a test for it!!!)
• fix the parsing logic to handle multivalue headers
• add an S3 test around multivalue headers (requests concatenates multivalue headers by default but urllib3 does not-
• add an order S3 test because my first intuition was related to the order of returned fields

Testing

If you remove the parser fix, the tests now fail.
I also validated that the user-provided sample in the linked issue now works 👍