Remove accept encoding header

[cloutierMat]

Motivation

Investigating issues with ApiGateway sending wrong Accept-Encoding headers to it's integration, with @bentsku, we found urllib3.util.SKIP_HEADER which let's us prevent requests from adding the specified header.

Changes

• Adds SKIP_HEADER instead of identity when the Accept-Encoding header isn't set
• Adds tests

[bentsku]

LGTM! This will allow us to remove some snapshot skips in upstream dependencies, and is a "more correct" way of handling the Accept-Encoding header. The identity workaround is known, but we can have better parity and identical behaviors with this.
TIL about this header, you already showed it to me in APIGW and it really helps! 🚀

I'll leave the final review to @alexrashed, but this has been proven to work in APIGW NextGen already 😄

[alexrashed]

Also looks good to me! Happy to merge this as it is!
Is this change already being awaited / should we create a release right away?

[cloutierMat]

As far as I know the only follow up upstream will be to cleanup some snapshot tests, unless @bentsku is actively working on those there is no rush from my part.