Return stream to pool on exact read (#509)

If the user reads exactly the number of bytes in the response, then
drops the Read object, streams will never get returned to the pool since
the user never triggered a read past the end of the LimitedRead.

This fixes that by making PoolReturnRead aware of the level below it, so
it can ask if a stream is "done" without actually doing a read.

Also, a refactorign:

Previously, Response contained an Option<Box<Unit>> because the testing
method `from_str()` would construct a Response with no associated Unit.
However, this increased code complexity with no corresponding test
benefit. Instead, construct a fake Unit in from_str().

Also, instead of taking an `Option<Box<Unit>>`, PoolReturnRead now takes
a URL (to figure out host and port for the PoolKey) and an &Agent where
it will return the stream. This cuts interconnectedness somewhat:
PoolReturnRead doesn't need to know about Unit anymore.

src/unit.rs

@@ -284,7 +284,7 @@ fn connect_inner(
     // TODO: this unit.clone() bothers me. At this stage, we're not
     // going to use the unit (much) anymore, and it should be possible
     // to have ownership of it and pass it into the Response.
-    let result = Response::do_from_stream(stream, Some(unit.clone()));
+    let result = Response::do_from_stream(stream, unit.clone());
 
     // https://tools.ietf.org/html/rfc7230#section-6.3.1
     // When an inbound connection is closed prematurely, a client MAY