Only persist 1.1 connections and clarify persist logic

HTTP::Tiny is an HTTP/1.1 client, so it doesn't need to support
HTTP/1.0 response headers.

The changes to the logic make it clearer that the condition for
persistance are having a known message length, as provided by
8.1.2.1 in the spec.
use keep_alive by default
Keep connection if some attributes are modified

Some attributes aren't connection related so we can keep persistence
for them.

Also documents keep alive behavior better.