Offload transparent decompression to browser

[zzz]

ref: #2190 and subsequent discussion

Following recent changes to offload compression to the web server and browser (by passing through the Accept-Encoding header), we could offload the currently-transparent client tunnel decompression (when compressed by the server tunnel) by tweaking the headers.

We'd have to remember the Accept-Encoding header in the request to ensure support, and convert the X-I2P-xxx header to a Content-Encoding header in the response. Also, I think the gzip header format may be different? If so would have to fix that up too.