fix Transfer-Encoding behavior in input for SimpleRequestsClient

[bentsku]

Motivation

This was reported in https://github.com/orgs/localstack/discussions/35
We were forwarding the Transfer-Encoding header back to the server, even though it is a hop-by-hop server. We have done a lot of work already to remove it from responses, but we did not do it in the SimpleRequestsClient

It was however done in the Proxy a while back.

See hop-by-hop headers: https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers

Hop-by-hop headers
These headers are meaningful only for a single transport-level connection, and must not be retransmitted by proxies or cached. Note that only hop-by-hop headers may be set using the Connection header.

Changes

• add a one line to remove any Transfer-Encoding header which shouldn't be forwarded
• add tests for it

[alexrashed]

Great change, great tests, great PR description outlining the changes and referring the spec! 🤩