Add Accept-Encoding header to fetch method

[gRegorLove]

Closes #208

[Zegnat]

I do not think we have to be known for fetching, or supporting all those cases. So I would lean towards not supporting retries. It is always possible for someone to feed the data into Mf2\parse after they have gotten it themselves.

It would be good to support both constants, as @jtojnar says, based on the data that PHP gives us:

1. CURLOPT_ENCODING: “Available as of cURL 7.10 and deprecated as of cURL 7.21.6.”
2. CURLOPT_ACCEPT_ENCODING: “Available as of cURL 7.21.6.”

Probably just based on curl_version(), using version_compare()? Again, unlike SimplePie, I would not try to make that logic any harder than it has to be.