From<http::Response> for Response (#360)

This adds an implementation to convert a `Response` type from the `http`
crate to the `async_impl::Response` type. This is the first step to
allow us to convert `http::Response` objects to `request::Response`
objects

This also adds an extension trait for the `http::response::Builder`
type. The `http::Response` object does not provide a way to access the
"final" url that the response is derived from, so we can't easily
provide that in the `From<http::Response>` implementation. For users who
are manually constructing `http::Response` objects for use in tests,
etc, they can import this extension trait, which adds a `.url()` builder
method that will allow them to pass a `Url`, which we then convert to
our newtype'd Url and add to the `http::Response`'s `extensions`. Then,
when converting from `http::Response` to `async_impl::Response` we can
pull that value out of the `extensions` and use it to construct the
`async_impl::Response`

Closes #333

src/response.rs

@@ -6,6 +6,7 @@ use std::borrow::Cow;
 
 use encoding_rs::{Encoding, UTF_8};
 use futures::{Async, Poll, Stream};
+use http;
 use mime::Mime;
 use serde::de::DeserializeOwned;
 use serde_json;
@@ -336,3 +337,9 @@ pub fn new(mut res: async_impl::Response, timeout: Option<Duration>, thread: Kee
     }
 }
 
+impl<T: Into<async_impl::body::Body>> From<http::Response<T>> for Response {
+    fn from(r: http::Response<T>) -> Response {
+        let response = async_impl::Response::from(r);
+        new(response, None, KeepCoreThreadAlive::empty())
+    }
+}