! httpx: decode should remove Content-Encoding header from message

This fixes #316 by changing Decoder.decode to remove the Content-Encoding header.

I added extra round-trip unit tests to the DeflateSpec and GzipSpec but I also added some basic unit tests for the Encoder and Decoder abstract classes themselves.
Merge pull request #385 from agemooij/issue-316

Fix for #316: ! httpx: decode should remove Content-Encoding header from message