Simplify Response object

[jsha]

Now that we create a Reader immediately on construction, we can get rid of unit, length, and compression by passing the necessary parameters directly to stream_to_reader. This also lets us avoid temporarily making a half-constructed Response so we can call stream_to_reader on it. Now stream_to_reader is an associated method that doesn't take Self so it can be called during construction.

Followup to #531.

[algesten]

@jsha In my mind, the whole reason for Unit is to encapsulate the request arguments. I think this PR does do a nice cleanup, but we could use Unit as an argument into stream_to_reader. I pushed a commit to that effect.

Reading the code there is another problem.

1. Here we remove content-length from headers
2. We pass headers to stream_to_reader
3. Then we try to parse content-length from headers in stream_to_reader.